Method and apparatus for generating a variable capacitance for synthesizing high-frequency signals for wireless communications

ABSTRACT

A method and apparatus for synthesizing high-frequency signals, such as wireless communication signals, includes a phase-locked loop (PLL) frequency synthesizer with a variable capacitance voltage controlled oscillator (VCO) that has a discretely variable capacitance in conjunction with a continuously variable capacitance. The discretely variable capacitance may provide coarse tuning adjustment of the variable capacitance to compensate for capacitor and inductor tolerances and to adjust the output frequency to be near the desired frequency output. The continuously variable capacitance may provide a fine tuning adjustment of the variable capacitance to focus the output frequency to match precisely the desired frequency output. The continuously variable capacitance may be formed by using a plurality of separate capacitance circuits. The individual capacitance circuits may include two capacitors coupled to a variable resistance element. The variable resistance element may be a transistor controlled by an analog control voltage. The total capacitance of the continuously variable capacitance may be substantially linear with respect to the phase of the VCO output while the individual capacitance circuits exhibit nonlinear behavior.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the synthesis ofhigh-frequency signals. More particularly, the present invention relatesto the synthesis of high-frequency local oscillator signals for wirelesscommunication applications.

BACKGROUND

Wireless communication systems typically require frequency synthesis inboth the receive path circuitry and the transmit path circuitry. Forexample, cellular phone standards in the United States and Europe definea cellular telephone system with communication centered in two frequencybands at about 900 MHz and 1800 MHz. For example, United States cellularphone standards include (1) the AMPS (analog), IS-54 (analog/digital),and IS-95 (analog/digital) standards in the 900 MHz frequency band, and(2) PCS (digital) standards in the 1800 MHz range. European cellularphone standards include (1) the TACS (analog) and GSM (digital)standards in the 900 MHz frequency band, and (2) the DCS 1800 (digital)standard in the 1800 MHz range. A dual band cellular phone is capable ofoperating in both the 900 MHz frequency band and the 1800 MHz frequencyband.

Within the frequency bands, the cellular standards define systems inwhich base station units and mobile units communicate through multiplechannels, such as 30 kHz (IS-54) or 200 kHz (GSM) wide channels. Forexample, with the IS-54 standard, approximately 800 channels are usedfor transmitting information from the base station to the mobile unit,and another approximately 800 channels are used for transmittinginformation from the mobile unit to the base station. A frequency bandof 869 MHz-894 MHz and a frequency band of 824 MHz-849 MHz are reservedfor these channels, respectively. Because the mobile unit must becapable of transmitting and receiving on any of the channels for thestandard within which it is operating, a frequency synthesizer must beprovided to create accurate frequency signals in increments of theparticular channel widths, such as for example 30 kHz increments in the800-900 MHz region.

Phase-locked loop (PLL) circuits including voltage controlledoscillators (VCOs) are often used in mobile unit applications to producethe desired output frequency (f_(OUT)). The output frequency may be madeprogrammable by utilizing an output frequency feedback divider (÷N) anda reference divider (÷R) for an input reference frequency (f_(REF)). Theoutput frequency produced is a function of the values selected for “N”and “R” in the divider circuits, such that f_(OUT)=N(f_(REF)/R). The PLLcircuitry typically utilizes a phase detector to monitor phasedifferences (Δθ) between the divided reference frequency (f_(REF)/R) andthe divided output frequency (f_(OUT)/N) to drive a charge pump. Thecharge pump delivers packets of charge proportional to the phasedifference (Δθ) to a loop filter. The loop filter outputs a voltage thatis connected to the VCO to control its output frequency. The action ofthis feedback loop attempts to drive the phase difference (Δθ) to zero(or at least to a constant value) to provide a stable and programmableoutput frequency.

The values for the reference frequency and the divider circuits may bechosen depending upon the standard under which the mobile unit isoperating. For example, within the United States IS-54 system, a PLLcould be built such that f_(REF)/R=30 kHz and such that N is on theorder of 30,000. The output frequency, therefore, could then be set in30 kHz increments to frequencies in the 900 MHz frequency band.Similarly, within the European GSM system, a PLL could be built suchthat f_(REF)/R=200 kHz and such that N is on the order of 4,500. Theoutput frequency, therefore, could then be set in 200 kHz increments tofrequencies in the 900 MHz frequency band.

The performance of the communication system, however, is criticallydependent on the purity of the synthesized high-frequency outputsignals. For signal reception, impure frequency sources result in mixingof undesired channels into the desired channel signal. For signaltransmission, impure frequency sources create interference inneighboring channels. A frequency synthesizer, therefore, must typicallymeet very stringent requirements for spectral purity. The level ofspectral purity required in cellular telephone applications makes thedesign of a PLL synthesizer solution and, in particular, the design of aVCO within a PLL synthesizer solution quite demanding.

Three types of spectral impurity will typically occur in VCO circuitsthat are used in PLL implementations for frequency synthesis: harmonicdistortion terms associated with output frequency, spurious tones nearthe output frequency, and phase noise centered on the output frequency.Generally, harmonic distortion terms are not too troublesome becausethey occur far from the desired fundamental and their effects may beeliminated in cellular phone circuitry external to the frequencysynthesizer. Spurious tones, however, often fall close to thefundamental. In particular, spurious tones at frequencies of ±f_(REF)/Rfrom the output frequency (f_(OUT)) are often found in the outputfrequency spectrum. These are called reference tones. Spurious tones,including reference tones, may be required by a cellular phoneapplication to be less than about −70 dBc, while harmonic distortionterms may only be required to be less than about −20 dBc. It is notedthat the “c” indicates the quantity as measured relative to the power ofthe “carrier” frequency, which is the output frequency.

Phase noise is undesired energy spread continuously in the vicinity ofthe output frequency, invariably possessing a higher power density atfrequencies closer to the fundamental of the output frequency. Phasenoise is often expressed as dBc/Hz or dBc/Hz. Phase noise is often themost damaging of the three to the spectral purity of the outputfrequency. Because of the effect phase noise has on system performance,a typical cellular application might require the frequency synthesizerto produce an output frequency having phase noise of less than about−110 dBc/Hz at 100 kHz from the output frequency.

Because the phase noise specifications are so stringent in cellularphone applications, the VCOs used in cellular phone PLL synthesizersolutions are typically based on some resonant structure. Ceramicresonators and LC tank circuits are common examples. While details inthe implementation of LC tank oscillators differ, the general resonantstructure includes an inductor (L) connected in parallel with a fixedcapacitor (C) and a variable capacitor (C_(X)). In the absence of anylosses, energy would slosh between the capacitors and the inductor at afrequency f_(OUT)=(½π)[L(C+C_(X))]^(−½). Because energy will bedissipated in any real oscillator, power in the form of a negativeconductance source, such as an amplifier, is applied to maintain theoscillation. It is often the case that the series resistance of theinductor is the dominant loss mechanism in an LC tank oscillator,although other losses typically exist.

While it is highly desirable to integrate the VCO with the othercomponents of the PLL onto a single integrated circuit for cost, size,power dissipation, and performance considerations, barriers tointegration exist. One of the more significant barriers is the lack ofprecision in the values of the inductors and capacitors used in the LCtank of the PLL. This tolerance problem typically forces most PLLsynthesizer implementations to modify the inductor or capacitor valuesduring production, for example, by laser trimming. Further complicatingintegration is the difficulty in integrating an inductor with a lowseries resistance and a capacitor with a reasonably high value and withlow loss and low parasitic characteristics. In integrating capacitancevalues, a significant problem is the high value of a typical loop filter(LF) capacitor component, which is often on the order of 1-10 μF.Another significant problem is the absence of a variable capacitance(C_(X)) component that possesses a highly-variable voltage-controlledcapacitance that is not also a high loss component that causes phasenoise. To provide this variable capacitance (C_(X)) component, ahigh-precision reverse-biased diode or varactor is typically utilized.However, such high-performance varactors require special processing and,therefore, have not been subject to integration with the rest of the PLLcircuitry. In short, although integration onto a single integratedcircuit of a PLL implementation for synthesizing high-frequency signalsis desirable for a commercial cellular phone application, integrationhas yet to be satisfactorily achieved.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and apparatus forsynthesizing high-frequency signals is disclosed that overcomes theintegration problem associated with prior implementations and meets thedemanding phase noise and other impurity requirements. The presentinvention achieves this advantageous result by implementing aphase-locked loop (PLL) frequency synthesizer with a variablecapacitance voltage controlled oscillator (VCO) that includes adiscretely variable capacitance in conjunction with a continuouslyvariable capacitance. The discretely variable capacitance may providecoarse tuning adjustment of the variable capacitance to compensate forcapacitor and inductor tolerances and to adjust the output frequency tobe near the desired output frequency. The continuously variablecapacitance may provide a fine tuning adjustment of the variablecapacitance to focus the output frequency to match precisely the desiredoutput frequency and to provide compensation for post-calibration driftof the PLL circuitry. The present invention avoids the need for atraditional varactor implementation in the VCO, for a traditional largecapacitor component in the loop filter, and for component trimmingduring processing and thereby provides a high-frequency frequencysynthesizer that may be fully integrated on a single chip except for anexternal inductor.

In one embodiment, a wireless communication frequency synthesizer havingat least one variable frequency output, the output frequency varying asa capacitance within the synthesizer varies, is provided. Thesynthesizer may include a variable capacitance circuit integrated withinthe frequency synthesizer. The variable capacitance circuit may comprisea first capacitor and a second capacitor coupled in series between afirst node and a second node, a control node coupled between the firstand second capacitors, and a variable resistance coupled between thecontrol node and the second node. The synthesize may also include avariable control signal coupled to the variable resistance element.

In another embodiment, a method of operating a wireless communicationfrequency synthesizer may comprise providing a capacitance circuitintegrated within the frequency synthesizer, the capacitance circuitcomprising a first capacitor and a second capacitor coupled in seriesbetween a first node and a second node, a third node being between thefirst and second capacitors. The method may further include providing avariable resistance coupled to the third node, generating a variablecontrol signal, and controlling the resistance of the variableresistance with the variable control signal to vary the capacitance ofthe capacitor circuit.

In another embodiment, a wireless communication frequency synthesizerhaving at least one variable frequency output, the output frequencyvarying as a capacitance within the synthesizer varies may comprise avariable capacitance circuit integrated within the frequencysynthesizer. The variable capacitance circuit may comprise a firstcapacitor coupled between a first node and a second node, a control nodecoupled between the first and second nodes, and a variable resistancecoupled between the control node and the second node. The synthesizermay also include an analog variable control signal coupled to thevariable resistance .

In still another embodiment, a method of operating a wirelesscommunication frequency synthesizer is provided. The method includesproviding a capacitance circuit integrated within the frequencysynthesizer, the capacitance circuit comprising a first capacitorcoupled between a first node and a second node, a third node beingbetween the first and second nodes. The method also includes providing avariable resistance coupled to the third node, generating a variableanalog control signal, and controlling the resistance of the variableresistance with the variable analog control signal to vary thecapacitance of the capacitance circuit.

A wireless communication frequency synthesizer having at least onevariable frequency output, the output frequency varying as a capacitancewithin the synthesizer varies is also provided. The synthesizer maycomprise a frequency output, and a substantially linear variablecapacitance circuit integrated within the frequency synthesizer. Thevariable capacitance circuit may comprise a plurality of nonlinearcapacitance circuits, at least some of the nonlinear capacitancecircuits controlled by at least one variable control signal. Thesubstantially linear variable capacitance circuit may have asubstantially linear relationship between the phase of the frequencyoutput and the capacitance of the variable capacitance circuit.

In still another embodiment, a method of operating a wirelesscommunication frequency synthesizer is provided. The method comprisesproviding an integrated variable capacitance circuitry, the variablecapacitance circuitry formed from a plurality of capacitance circuits,adjusting a capacitance of each of the plurality of capacitance circuitsin a nonlinear manner, and adjusting the output frequency of thesynthesizer by varying a total capacitance of the variable capacitancecircuitry in a substantially linear manner.

DESCRIPTION OF THE DRAWINGS

It is noted that the appended drawings illustrate only exemplaryembodiments of the invention and are, therefore, not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1 is a block diagram of receive path circuitry for a wirelesscommunication device, such as a mobile unit in a cellular phone system.

FIG. 2 is a block diagram of phase-locked loop (PLL) circuitry forsynthesizing frequencies required by the frequency synthesizer in FIG.1.

FIG. 3 (Prior Art) is a block diagram of a typical prior artimplementation for a LC tank voltage controlled oscillator (VCO) withinthe PLL depicted in FIG. 2.

FIG. 4 depicts a general circuit diagram of a digital and analog VCOimplementation according to the present invention.

FIG. 5 is a block diagram of a frequency synthesizer that takesadvantage of a digital and analog VCO implementation according to thepresent invention.

FIG. 6A is a block diagram of an integrated circuit (IC) according tothe present invention that may provide frequency synthesis for a dualband mobile phone application.

FIG. 6B is a block diagram of an alternative embodiment according to thepresent invention for the integrated circuit (IC) depicted in FIG. 6Athat provides dual frequency bands with a single RF frequency PLL.

FIG. 7 is a circuit diagram of an embodiment for discretely variablecapacitance circuitry according to the present invention.

FIG. 8 is a block diagram of an embodiment for discrete controlcircuitry according to the present invention for providing a digitalcontrol word to the discretely variable capacitance circuitry of FIG. 7.

FIGS. 9A, 9B and 9C are circuit diagrams depicting parasiticcapacitances associated with the components in FIG. 7 and a transistormodification for alleviating problems associated with these parasiticcapacitances.

FIG. 10 is a block diagram of a differential embodiment according to thepresent invention for the VCO depicted in FIG. 4.

FIG. 11 is a circuit diagram of a differential amplifier according tothe present invention for the differential embodiment depicted in FIG.10.

FIG. 12 is a circuit diagram depicting transistors that may be added tothe differential embodiment of FIG. 10 to improve the performance ofcapacitor pairs within the discretely variable capacitance circuitry.

FIG. 13 is a diagram of an embodiment of a VCO for achieving dual bandoperation for the VCO depicted in FIG. 6B.

FIG. 14 is a block diagram of an alternative embodiment of a VCO forachieving dual band operation for the VCO depicted in FIG. 6B using twoVCOs within a single PLL circuit.

FIG. 15 is a block diagram of a frequency synthesizer utilizing adigital and analog VCO implementation according to the presentinvention.

FIG. 16 is a block diagram of the analog control loop of the frequencysynthesizer of FIG. 15.

FIGS. 17A, 17B, 17C and 17D are circuit diagrams of embodiments of acontinuously variable capacitor circuit.

FIG. 18 is a capacitance verses conductance graph of the circuit of FIG.17C.

FIG. 19 is an alternative embodiment of a continuously variablecapacitor circuit.

FIG. 20 is a shift register timing diagram.

FIG. 21 is a function block diagram of a phase detector/sample holdcircuit.

FIG. 22 is a diagram of a phase detector/sample hold circuit.

FIG. 23 is a voltage generator for use with the circuit of FIG. 22.

FIGS. 24A and 24B are timing diagrams of a voltage node of the circuitof FIG. 22.

FIG. 24C is a timing diagram for the circuit of FIG. 22.

FIG. 25 is a block diagram of an implementation of a plurality of thecircuits of FIG. 22.

FIGS. 26-31 are diagrams of the transfer function characteristics of theanalog control techniques.

FIG. 32 is a circuit diagram for an alternative method of generating aplurality of VCO control voltages.

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates a method and apparatus forsynthesizing high-frequency signals by implementing a phase-locked loop(PLL) frequency synthesizer with a variable capacitance voltagecontrolled oscillator (VCO) that includes a discretely variablecapacitance in conjunction with a continuously variable capacitance. Inparticular, the frequencies synthesized by the present invention may beused in receive and transmit path circuitry for wireless communicationdevices.

FIG. 1 is a block diagram of receive path circuitry 150 for a wirelesscommunication device, such as a mobile unit in a cellular phone system.An incoming signal is received by the antenna 108, filtered by aband-pass filter 110, and amplified by a low noise amplifier 112. Thisreceived signal is typically a radio-frequency (RF) signal, for examplea 900 MHz or 1800 MHz signal. This RF signal is usually mixed down to adesired intermediate frequency (IF) before being mixed down to baseband.Using a reference frequency (f_(REF)) 106 from a crystal oscillator 105,frequency synthesizer 100 provides an RF mixing signal (RF_(OUT)) 102 tomixer 114. Mixer 114 combines this RF_(OUT) signal 102 with the filteredand amplified input signal (RF_(IN)) 113 to produce a signal 115 thathas two frequency components represented by |RF_(IN)+RF_(OUT)| and|RF_(IN)−RF_(OUT)|. The signal at the latter of these two is selected byband-pass filter 116 to provide an IF signal 117. This IF signal 117 isthen amplified by variable gain amplifier 118 before being mixed down tobaseband by mixers 122 and 124.

Signal processing in mobile phones is typically conducted at basebandusing in-phase (I) and quadrature (Q) signals. The Q signal is offsetfrom the I signal by a phase shift of 90 degrees. To provide these twosignals, an IF mixing signal (IF_(OUT)) 104 and a dual divide-by-two andquadrature shift block (÷2/90°) 120 may be utilized. Frequencysynthesizer 100 generates an IF_(OUT) signal 104, for example at about500 MHz, that is divided by 2 in block 120 to provide IF_(OUT)/2 mixingsignals 119 and 121. Block 120 delays the signal 121 to mixer 122 by 90degrees with respect to the signal 119 to mixer 124. Block 120 may beimplemented with two flip-flop circuits operating off of opposite edgesof the IF_(OUT) signal 104, such that the output of the flip-flops arehalf the frequency of the IF_(OUT) signal 104, and are 90 degrees offsetfrom each other. The resulting output signals 123 and 125 have twofrequency components represented by |IF+IF_(OUT)/2| and |IF−IF_(OUT)/2|.The latter frequency component is the desired one and is typicallyselected such that the baseband signal is centered at DC (f=0 Hz).Assuming the baseband frequency is centered at DC, the |IF−IF_(OUT)/2|signal is selected using low-pass filters 126 and 128. The resultingbaseband signal 123 is the Q signal, and the resulting baseband signal125 is the I signal. These signals 123 and 125 may be further processedat baseband by processing block 130 and provided to the rest of themobile phone circuitry as I and Q signals 131 and 132.

FIG. 2 is a block diagram of phase-locked loop (PLL) circuitry 200 forsynthesizing one of the frequencies required by frequency synthesizer100. A second PLL 200 may be implemented to provide the secondfrequency. The reference frequency (f_(REF)) 106 is received by adivide-by-R (÷R) counter 204, and the output frequency (f_(OUT)) 102 isreceived by a divide-by-N (÷N) counter 214. The resulting dividedsignals (f_(φR)) 216 and (f_(φN)) 218 are received by a phase detector(PD) 206. The PD 206 determines the phase difference (Δθ) between thephase (θ_(φR)) of the divided signal 216 and the phase (θ_(φN)) of thedivided signal 218. The PD 206 uses this phase difference (Δθ) to drivea charge pump (CP) 208. The CP 208 provides a voltage output that isfiltered by a loop filter 210 to provide a voltage control (V_(C))signal 220. The V_(C) signal 220 controls the output frequency (f_(OUT))102 of a voltage controlled oscillator (VCO) 212. The values for N and Rmay be selected to provide a desired output frequency such thatf_(OUT)=N(f_(REF)/R). For a typical mobile phone application, theIF_(OUT) frequency 104 will remain constant, while the RF_(OUT)frequency 102 will change depending upon the channel of the incomingsignal. Thus, a first PLL may be used to provide the IF_(OUT) frequency104, and its N and R values may be programmed once and then left alone.A second PLL may be used to provide the RF_(OUT) frequency 102, and itsN and R values may be selectively programmed to provide the desiredRF_(OUT) signal 102. If desired, the R value for this second PLL may beprogrammed once and left alone, while the N value may be used to selectthe desired RF_(OUT) signal 102.

The transmit path circuitry (not shown) for a wireless communicationdevice, such as a mobile unit in a cellular phone system, may includecircuitry to move the outgoing signal from baseband to an RFtransmission frequency. A transmit frequency band for cellular phonesystems typically includes the identical number of channels as includedwithin the receive frequency band. The transmit channels, however, areshifted from the receive channels by a fixed frequency amount. In such asystem, a cellular phone application may utilize the RF mixing signal(RF_(OUT)) 102 synthesized by the frequency synthesizer 100 for a givenchannel in both the receive path and the transmit path circuitry. Forexample, if the frequency synthesizer 100 has been designed as part ofthe receive path circuitry 150, the RF mixing signal (RF_(OUT)) 102 fora given channel within the receive frequency band may be shifted by thefixed frequency amount to provide a desired RF mixing signal to thetransmit path circuitry. Alternatively, the frequency synthesizer 100may be designed as part of the transmit path circuitry, or two separatefrequency synthesizers 100 may be utilized.

FIG. 3 (Prior Art) is a block diagram of a typical prior artimplementation for VCO 212 using an LC tank oscillator and varactor 312.As also discussed above, the typical use of a varactor in cellular phoneapplications has been a major factor in limiting the integration of PLLcircuitry 200 into a single chip. Looking to FIG. 3 (Prior Art), anexternal inductor (L_(EXT)) 302 and an external capacitor (C_(EXT)) 304are connected in parallel with a variable capacitance (C_(X)) 306 and anegative conductance source (−G) 314. Because there will be some losseswithin the VCO, negative conductance source (−G) 314 is provided as anactive device that adds back energy lost to sustain oscillation. Thevariable capacitance (C_(X)) 306 is implemented using a fixed couplingcapacitor (C_(C)) 308 connected in series with a varactor (D_(VAR)) 312.Varactor (D_(VAR)) 312 is a reverse-biased diode that has a capacitancevariable depending upon the reverse-bias voltage applied at the voltagecontrol node (V_(C)) 220. This node (V_(C)) 220 is connected between thecoupling capacitor (C_(C)) 308 and the variable diode (D_(VAR)) 312through a coupling resistor (R_(C)) 310. The value of the couplingresistor (R_(C)) 310 is chosen to be large so that it is effectively anopen circuit at high frequencies (i.e., near the frequency ofoscillation). The variable capacitance (C_(X)) 306 is the seriescombination of the coupling capacitor (C_(C)) 308 and thevoltage-controlled capacitance of the varactor (D_(VAR)) 312. The outputoscillation frequency (f_(OUT)) 102 is thereby made to be a function ofthe voltage control node (V_(C)) 220, such thatf_(OUT)=(½π)[L_(EXT)(C_(EXT)+C_(X)(V_(C)))]^(−½).

As discussed above, it is desirable for the PLL circuitry 200 to beintegrated onto a single chip. As also discussed above, however, priorto the present invention, commercial cellular phone applications havebeen limited to integration of only parts of the circuit portions withinthe PLL circuitry 200. For example, the dotted line 202 depicted in FIG.2 represents the portions of the PLL circuitry 200 that have beenintegrated into a single integrated circuit. The present invention,however, provides a frequency synthesis solution that is capable of fullintegration while still providing high fidelity high-frequency signals.The present invention is now described in general aspects with respectto FIGS. 4 and 5.

FIG. 4 depicts a general circuit diagram of a VCO 400 according to thepresent invention that avoids problems associated with prior artdesigns. The VCO 400 produces an output frequency (f_(OUT)) 102 using anLC tank oscillator having an external inductor (L_(EXT)) 302. Theexternal capacitor (C_(EXT)) 304 represents any desired externallyconnected capacitance and the parasitic capacitance of the semiconductordevice leads. Unlike the prior art, the present invention achieves avariable capacitance (C_(X)) 401 with a discretely variable capacitance(C_(D)) 402 in conjunction with a continuously variable capacitance(C_(A)) 406. The discretely variable capacitance (C_(D)) 402 may becontrolled by a digital control word (B_(C)) 404, and the continuouslyvariable capacitance (C_(A)) 406 may be controlled by a voltage controlsignal (V_(C)) 408. It is noted that the digital control word (B_(C))404 and the voltage control signal (V_(C)) 408 may be a single signal ora plurality of signals, as desired, depending upon the implementationfor the discretely variable capacitance (C_(D)) 402 and the continuouslyvariable capacitance (C_(A)) 406. A fixed capacitance (C_(F)) 410represents internal parasitic capacitance along with any desired fixedcapacitance connected internally to the integrated circuit. A negativeconductance source (−G) 314 is also provided to take care of losses inthe VCO 400.

In operation, the discretely variable capacitance (C_(D)) 402 may beused after manufacture to dynamically compensate for any componenttolerance problems including all of the internal capacitance values, anyexternal capacitor (C_(EXT)) 304, and the external inductor (L_(EXT))302. In addition, the discretely variable capacitance (C_(D)) 402 may beused to provide coarse tuning of the desired output frequency, therebyreducing the frequency range that must be covered by variations in thecapacitance of the continuously variable capacitance (C_(A)) 406. Aftercoarse tuning by the discretely variable capacitance (C_(D)) 402, thecontinuously variable capacitance (C_(A)) 406 may be used to providefine tuning of the desired output frequency. This coarse and fine tuninginitially calibrates the output frequency (f_(OUT)) 102 to the desiredoutput frequency. After the initial calibration, the continuouslyvariable capacitance (C_(A)) 406 may be used to compensate for anypost-calibration frequency drift. Such post-calibration frequency driftwill typically occur due to a variety of factors, including for exampletemperature variations. In this way, the present invention allows forthe VCO 400 to be manufactured without the trimming requirements ofprior implementations and allows a high-frequency PLL frequencysynthesizer to be integrated on a single integrated circuit. Inparticular, the high-frequency PLL frequency synthesizer of the presentinvention provides an output frequency having phase noise of less thanabout −110 dBc/Hz at 100 kHz from the output frequency.

An example will now be provided for the coarse and fine tuning that maybe provided by a VCO 400 according to the present invention. Asdescribed above, the United States IS-54 standard utilizes on the orderof eight hundred 30 kHz wide channels in a frequency band of 869 MHz-894MHz for transmitting information from a base station to a mobile unit.One receive channel may be for example at 870.03 MHz. Assuming that acellular phone application has been designed to have an IF frequency of250 MHz, the RF mixing frequency that must be synthesized by thefrequency synthesizer for this channel would need to be 1120.03 MHz. (Itis noted that for the 900 MHz frequency band, the RF mixing frequencyutilized is typically above the channel frequency, although an RF mixingfrequency below the channel frequency may also be used.) The discretelyvariable capacitance (C_(D)) 402 may be designed to coarsely tune the RFoutput frequency of the frequency synthesizer to about 0.1% of thedesired frequency of 1120.03 MHz or to within about 1 MHz. Thecontinuously variable capacitance (C_(A)) 406 may be designed to providea frequency range of about 1% of the desired frequency of 1120.03 MHz ora range of about 11 MHz, which is about 10 times the coarse tuningaccuracy of the discretely variable capacitance (C_(D)) 402. Thisfrequency range allows the continuously variable capacitance (C_(A)) 406to finely tune the RF output frequency of the frequency synthesizer tothe desired frequency of 1120.03 MHz and to compensate forpost-calibration frequency drift. The initial voltage input values forthe continuously variable capacitance (C_(A)) 406 may be selected sothat the continuously variable capacitance (C_(A)) 406 may move the RFoutput frequency either up or down by roughly the same amount.

FIG. 5 is a block diagram of a frequency synthesizer 500 that takesadvantage of a digital and analog VCO 400 according to the presentinvention. The input reference frequency (f_(REF)) 106 is received bythe divide-by-R (÷R) counter 204. The output frequency (f_(OUT)) 102 isreceived by the divide-by-N (÷N) counter 214. The discrete control block502 receives the divided output frequency (f_(OUT)/N) 218 and thedivided reference frequency (f_(REF)/R) 216, and the discrete controlblock 502 outputs a digital control word (B_(C)) to the digital andanalog VCO 400. The phase detector (PD) 206 compares the phasedifference between the divided output frequency (f_(OUT)/N) 218 and thedivided reference frequency (f_(REF)/R) 216 and provides signals to thecharge pump (CP) 208 that depends upon this phase difference. The outputof the charge pump (CP) 208 is filtered by the loop filter (LF) 210 toprovide a first control voltage node 508. Initial voltage generatorblock (V_(INIT)) 504 provides a second control voltage node 510. Aswitch (SW) 512 allows for selection of control voltage node 510 as thevoltage node to be provided to the voltage control (V_(C)) input 408 tothe digital and analog controlled VCO 400.

When PLL 500 initiates, control of the output frequency (f_(OUT)) 102lies with discrete control block 502. The switch 512 selects the initialvoltage node 510 as the voltage control for the voltage control (V_(C))input 408. The voltage control (V_(C)) is used as the control voltagefor the continuously variable capacitance (C_(A)) 406 within the digitaland analog controlled VCO 400. In addition to providing a voltage inputto the voltage control (V_(C)) input 408, this connection also chargesthe capacitors within the loop filter (LF) 210 to an initial voltagevalue. The discrete control block 502 includes digital logic that willdetermine through a desired procedure how to adjust the discretelyvariable capacitance (C_(D)) 402 to coarsely tune the output frequency(f_(OUT)) 102. This determination may depend for example upon acomparison of the reference frequency (f_(REF)) 106 to the outputfrequency (f_(OUT)) 102 or a comparison of the divided referencefrequency (f_(REF)/R) 216 to the divided output frequency (f_(OUT)/N)218. Depending upon the determination made, the discrete control block502 may adjust the digital control word (B_(C)) 404. The digital controlword (B_(C)) 404 is used to provide control signals to the discretelyvariable capacitance (C_(D)) 402 within the digital and analogcontrolled VCO 400.

Once the discrete control block 502 completes its coarse tuningprocedure, the discrete control block 502 may fix the digital controlword (B_(C)) 404 and then assert the START signal 506 to change switch(SW) 512 so that it deselects the control node 510. At this point, thecontrol voltage node 508 supplies the voltage to the control voltage(V_(C)) node 408. The divide-by-R (÷R) and divide-by-N (÷N) counters 204and 214 are reset with the zero-phase restart (ZPR) signal 505. Thezero-phase restart (ZPR) signal 505 presets the counters within thedivide-by-R (÷R) and divide-by-N (÷N) counters 204 and 214 so that theinitial phase error is as small as possible when the first analog loopbecomes operable. From this point, the output frequency (f_(OUT)) 102 isfine tuned by the continuously variable capacitance (C_(A)) 406 throughoperation of phase detector (PD) 206, the charge pump (CP) 208 and theloop filter (LF) 210. If desired, the discrete control 502 may continueto monitor the output frequency (f_(OUT)) 102. If too great of an erroris detected, discrete control 502 may move the switch (SW) 512 back toselect initial control node 510 and again modify the digital controlword (B_(C)) 404 based upon a desired procedure.

In the embodiment depicted, therefore, only one control loop, eitherdigital or analog, is tuning the output frequency (f_(OUT)) 102 at anygiven moment. Initially, when the output frequency (f_(OUT)) 102 islikely far from the desired frequency, the digital control loop isoperable and the output frequency (f_(OUT)) 102 is modified by thedigital control word (B_(C)) 404 provided by the discrete control block502. When the discrete control block 502 completes its coarse tuningprocedure, the discrete control block 502 may assert the START signal506, thereby starting the action of the analog loop by setting theswitch (SW) 512 to deselect the initial voltage generator block(V_(INIT)) 504 and pass control to the voltage control node 508. At thispoint, the analog loop begins fine tuning the output frequency (f_(OUT))102 until a stable output frequency is reached. To allow thecontinuously variable capacitance (C_(A)) 406 within the analog loop tomove the output frequency (f_(OUT)) 102 either faster or slower inroughly equal amounts, the voltage value provided by the initial voltagegenerator block (V_(INIT)) 504 may be selected to be within the middleof the voltage range that may be provided by the control voltage node508 from the loop filter (LF) 210. It is also noted that if desired, anembodiment could be implemented in which both the digital and analogcontrol loops are active at the same time.

Further details of the present invention as utilized in a cellular phoneapplication will now be described. In particular, an overall blockdiagram for a dual band (900 MHz and 1800 MHz) cellular phoneapplication is described with respect to FIGS. 6A and 6B.

FIG. 6A is a block diagram of an integrated circuit (IC) 600 accordingto the present invention that may provide frequency synthesis for a dualband (e.g., 900 MHz and 1800 MHz) cellular phone application. The IC 600communicates with external control circuitry through serial interfacecircuitry 606, which may have for example an enable signal (EN_bar) pin,a serial data input (SDATA) pin, and a serial clock input (SCLK) pin.(It is noted that the suffix “_bar” is used to denote a signal that istypically asserted when at a low logic level.) Serial interfacecircuitry 606 may also include an internal shift register in which dataand command bits may be stored. This register may be for example, a22-bit shift register that may be serially loaded through the externalpin connections. The serial interface circuitry 606 communicates withthe rest of the circuitry through internal bus 605.

Other external pin connections for IC 600 may also include a power downcontrol (PDN_bar) pin connected to power down control circuitry 604 andclock input (XIN) and output (XOUT) pins connected to referenceamplifier circuitry 602. As depicted, the reference amplifier circuitry602 may, if desired, include a compensation digital-to-analog converter(DAC) register that controls variable capacitances (C_(V)). In thisembodiment, a crystal resonator may be connected between clock input(XIN) and output (XOUT) pins to complete the circuit, and the output ofthe reference amplifier circuitry 602 is the reference frequency(f_(REF)), which is used for synthesizing the desired outputfrequencies. Alternatively, the clock input (XIN) pin may be directlyconnected to receive the reference frequency (f_(REF)) from anoscillator circuit that has its own compensation circuitry, as depictedin FIG. 1. In this alternative embodiment, the output (XOUT) pin wouldnot need to be used.

The IC 600 provides the RF output frequency and the IF output frequencyneeded to mix the incoming RF signal to an IF frequency and then tobaseband. These frequencies are available through an RF output pin(RFOUT), which is connected to an output buffer 624, and an IF outputpin (IFOUT), which is also connected to an output buffer 642. Theseoutput frequencies are synthesized by the RF1 synthesizer, the RF2synthesizer, and the IF synthesizer. To provide a dual band solution,the IC 600 is able to synthesize RF output frequencies in two signalbands through RF1 synthesizer and the RF2 synthesizer. An RF select bit(RFSEL bit) 622 communicated through the serial interface circuitry 606is used to control multiplexer 618 to select either the RF1 synthesizeroutput or the RF2 synthesizer output. The RF select bit (RFSEL bit) 622is also used to control multiplexer 620 to select either the RF1 outputpower level setting 616 or the RF2 output power level setting 632.

The RF1 synthesizer may include reference divider circuitry 608, outputdivider circuitry 614, phase detector/loop filter 610, and a VCO 612.The reference divider circuitry 608 may include a register for thedivide-by-R value and divide-by-R circuitry. The output dividercircuitry 614 may include a register for the divide-by-N value, apre-scaler and counter circuitry (Swallow A Counter; N_(P) Counter), asis well known to those of skilled in the art. The phase detector/loopfilter 610 may include a phase detector gain register, phase comparatorcircuitry (φ), and filter circuitry (LF). The registers may be loadedthrough the internal bus 605 with data received through the serialinterface circuitry 606. The VCO 612 has connections for an externalinductor (RFLA/B), which may be selected to provide an output for theRF1 synthesizer in a desired frequency band.

The RF2 synthesizer may include reference divider circuitry 626, outputdivider circuitry 634, phase detector/loop filter 628, and a VCO 630.The reference divider circuitry 626 may include a register for thedivide-by-R value and divide-by-R circuitry. The output dividercircuitry 634 may include a register for the divide-by-N value, apre-scaler and counter circuitry (Swallow A Counter; N_(P) Counter), asis well known to those of skilled in the art. The phase detector/loopfilter 628 may include a phase detector gain register, phase comparatorcircuitry (φ), and filter circuitry (LF). The registers may be loadedthrough the internal bus 605 with data received through the serialinterface circuitry 606. The VCO 630 has connections for an externalinductor (RFLC/D), which may be selected to provided an output for theRF2 synthesizer in a desired frequency band that may be different fromthe frequency band of the RF1 synthesizer.

The IF synthesizer may include reference divider circuitry 636, outputdivider circuitry 644, phase detector/loop filter 638, and a VCO 640.The reference divider circuitry 636 may include a register for thedivide-by-R value and divide-by-R circuitry. The output dividercircuitry 644 may include a register for the divide-by-N value, apre-scaler and counter circuitry (Swallow A Counter; N_(P) Counter), asis well known to those of skilled in the art. The phase detectorfloopfilter 638 may include a phase detector gain register, phase comparatorcircuitry (φ), and filter circuitry (LF). The registers may be loadedthrough the internal bus 605 with data received through the serialinterface circuitry 606. The VCO 640 has connections for an externalinductor (IFLA/B), which may be selected to provide an output for the IFsynthesizer in a desired frequency range. As with the RF1 and RF2synthesizers, the IF synthesizer also has an output buffer 642 thatreceives an IF output voltage level setting 646.

It is understood that the embodiment depicted in FIG. 6A is an exampleembodiment and that modifications could be made to the design withoutdeparting from the present invention. The RF1, RF2, and IF synthesizersmay be implemented utilizing the PLL depicted in FIG. 5 and the VCOdepicted in FIG. 4. Possible alternative implementations to FIG. 6A arenow described with respect to FIG. 6., FIG. 13 and FIG. 14.

FIG. 6B is a block diagram of an alternative embodiment for theintegrated circuit (IC) 600 depicted in FIG. 6A. This alternativeembodiment also provides frequency synthesis for a dual band mobilephone applications, but does so with a single RF synthesizer. Theadvantageous result is achieved by implementing the VCO 612 with thecapability of switching between two output frequency bands. In so doing,the reference divider 608, the output divider 614, the phasedetector/loop filter 610, and the power output level setting 616 operateto synthesize frequencies in both bands without a change in circuitry.The programmable nature of the divider circuits 608 and 614 and thephase detector/loop filter 610 allows a selection of the desiredoperating parameters. The dual band VCO 612 provides output oscillationfrequencies in two different desired frequencies bands.

FIG. 13 is a diagram of an embodiment 1300 for achieving a dual bandoperation for VCO 612. The VCO 612 receives a voltage control (V_(C))signal 408 and provides an RF output (RF_(OUT)) frequency 102. Theexternal inductor (L_(EXT)) 1306 may be used to determine a first RFoutput frequency RF1. If a second RF output frequency RF2 is desired,NMOS transistor 1304 may be turned on through the assertion of a highlogic level on control node (CTRL) 1302. When this occurs, an additionalinductor (L_(PAR)) 1308 will be connected in parallel with the externalinductor (L_(EXT)) 1306. In this manner, the output frequency may beselectively centered in two desired bands of frequencies. As theinductance changes, the center frequency of oscillation of the LC tankwith the VCO 612 will also change. This approach may also be used toimplement any desired number of frequency bands by adding additionalswitches and inductances. Disadvantages to this approach include thelarge tolerances associated with most inductors and the undesirablephase noise added to the output frequency signals by transistor switch1304.

FIG. 14 is a block diagram of an alternative embodiment 1400 forachieving dual band operation for VCO 612. A second switch (SW) 1410 isused to select either a first VCO (VCO1) 612A or a second VCO (VCO2)612B. A first external inductor (L_(RF1)) 1416 may be selected so thatthe VCO1 612A has an RF output (RF1) 1412 centered in a first desiredfrequency band. Similarly, a second external inductor (L_(RF2)) 1418 maybe selected so that the VCO2 612B has an RF output (RF2) 1414 centeredin a second desired frequency band. The selected frequency is connectedthrough switch 1410 to provide the desired output frequency (f_(OUT))102. Power to the non-used VCO 612A or 612B may be shut down, forexample by starving the circuit of current from a current generator.This multiple VCO arrangement according to the present inventioneliminates potential sources of phase noise by moving the switch (SW)1410 outside of the LC tank. This approach may also be used to implementany desired number of frequency bands by adding additional VCOs,inductors, and switches (or multiplexers).

Further details of the discretely variable capacitance (C_(D)) 402 willnow be described. In particular, an implementation for the discretelyvariable capacitance (C_(D)) 402 is described with respect to FIG. 7,and an implementation for the discrete control block 502 as describedwith respect to FIG. 8.

FIG. 7 is a circuit diagram of an embodiment for discretely variablecapacitance (C_(D)) 402 according to the present invention. A fixedcapacitor (C_(F)) 410 represents parasitic capacitance plus any desiredfixed capacitance. Discrete variations are achieved through a pluralityof capacitor and transistor circuits connected together. Looking to thefirst of these connected circuits, an initial capacitor 702 (C_(D0)) isconnected between ground 412 and the signal line 414 through the drainand source terminals of an NMOS transistor 710. NMOS transistor 710 actsas a switch (S₀) to add in or leave out the capacitor (C_(D0)) 702 inthe overall capacitance of the discretely variable capacitance (C_(D))402. The “on” or “off” state of NMOS transistor 712 is controlled byfirst bit (B₀) 722 of a digital control word 404. Similarly, additionalcapacitors 704, 706 and 708 (C_(D1) . . . C_(DN−1), C_(DN)) may beconnected to additional NMOS transistors 712, 714 and 716 to form aplurality of connected capacitor circuits. The NMOS transistors 712, 714and 716 act as switches (S₁ . . . S_(N−1), S_(N)) and are controlled bybits 724, 726 and 728 (B₁ . . . B_(N−1), B_(N)) of a digital controlword 404.

Although impractical to implement off-chip, this digitally controlledarrangement may be reasonably integrated onto a single chip. Advantagesof this arrangement include providing a large range of possiblecapacitance variations and a solution to problems with poor componenttolerances. Another significant advantage is that it drastically reducesthe capacitance variation needed for the continuously variablecapacitance (C_(A)) 406. The discretely variable capacitance (C_(D)) 402may be used to provide a coarse tuning of the oscillation frequency nearthe desired output frequency. The continuously variable capacitance(C_(A)) 406 then needs only to vary enough to cover the frequency rangebetween the steps available through the discrete nature of the digitallycontrolled capacitance (C_(D)) 402 and to cover any component driftafter calibration, for example, due to temperature variations. Thisreduction in the required capacitance variation translates toeliminating the need for a large capacitance variation, which typicallyrequires the use of a variable reverse-biased diode (or varactor) asdescribed with respect to FIG. 3 (Prior Art) above. By eliminating theneed for this varactor, the present invention provides a frequencysynthesis solution that may be integrated on a single CMOS integratedcircuit.

It is noted that any desired number of capacitor/switch circuits may beconnected together as desired. It is also noted that numerous variationscould be made to the circuit depicted in FIG. 7 and still achieve acapacitance that is discretely variable based upon a digital controlword. The values of the capacitors and the control procedure implementedby the discrete control block 502 would depend upon the choices made.

For the circuit depicted in FIG. 7 with simple capacitor/switch circuitsconnected together in parallel, the total capacitance for the discretelyvariable capacitance (C_(D)) 402 is equal to the sum of all of thecapacitors having respective switches in the “on” state. Thus, the totalcapacitance for the discretely variable capacitance (C_(D)) 402 may berepresented by C_(D)=(C_(D0)·B₀)+(C_(D1)·B₁)+ . . .+(C_(DN−1)·B_(N−1))+(C_(DN)·B_(N)). If each capacitance value isconsidered a multiple of some unit or base capacitance value (C₀) timessome desired capacitor weighting (W), the total capacitance may berepresented by C_(D)=(W_(D0)C₀·B₀)+(W_(D1)C₀·B₁)+ . . .+(W_(DN−1)C₀·B_(N−1))+(W_(DN)C₀·B_(N)). In this embodiment, therefore,the choice of weighting coefficients defines what capacitances areavailable.

Numerous weighting schemes are possible, and the one implemented dependsupon the particular design considerations involved. One possible choicefor a weighting scheme is an equal weighting scheme, such that all ofthe weights are the same (W_(D0−N)=constant). This equal weightingscheme, however, is relatively inefficient because it requires a largenumber capacitor/switch circuits and a small base capacitor value toprovide a large number of capacitor value choices. Another possibleweighting scheme is a binary weighting scheme, such that each weight isa factor of 2 different from the previous weight (W_(D0)=1, W_(D1)=2,W_(D2)=4 . . . W_(DN−1)=2^(N−1), W_(DN)=2^(N)). Although this binaryweighting scheme is relatively efficient in allowing the selection of awide range of capacitance values with a limited number ofcapacitor/switch circuits, this scheme suffers from practicalimplementation problems due to differential non-linearities (DNL) inmanufacturing the capacitance values. The equal weighting scheme has alow occurrence of problems with DNL. Possible compromise weightingschemes between the equal and binary weighting schemes include a radixless-than-two and mixed radix weighting schemes. A radix less-than-twoweighting scheme, for example, may be implemented such that each weightis a factor (i.e., the radix) less than 2 (e.g., 7/4) different from theprevious weight (W_(D0)=1, W_(D1)=7/4, W_(D2)=(7/4)² . . .W_(DN−1)=(7/4)^(N−1), W_(DN)=(7/4)^(N)). A mixed radix weighting scheme,for example, may be implemented such that each weight is somecombination of factors (e.g., 2 and 7/4) different from the previousweight (W_(D0)=1, W_(D1)=2, W_(D2)=4, W_(D3)=4(7/4), W_(D3)=4(7/4)² . .. W_(DN)=2^(X)(7/4)^(Y) where X and Y are integers).

In TABLE 1 below, an example for the relative capacitor weighting valuesare set forth for a circuit as depicted in FIG. 7 in which the number ofcapacitor/switch circuits has been selected as eleven. The number ofbits in the digital control word (B_(C)) 404 has also been chosen to beeleven. This weighting scheme most closely resembles the mixed radixweighting scheme discussed above. It is noted that the weighting schemechosen will depend upon the circuit utilized and the coarse tuningalgorithm chosen. TABLE 1 below was selected for a dual band cellularphone application as depicted and described with respect to FIGS. 6A and6B above.

TABLE 1 Example Relative Capacitor Weightings CAPACITOR (C[N])WEIGHTINGS (W[N]) C[0] 1 C[1] 2 C[2] 4 C[3] 5 C[4] 10 C[5] 15 C[6] 30C[7] 50 C[8] 90 C[9] 160 C[10] 310

As discussed above, the selection of which of the capacitors in FIG. 7are added into the total output is determined by the digital controlword (B_(C)) 404.

As depicted in FIG. 5, the discrete control block 502 provides thedigital control word (B_(C)) 404 as an output. The discrete controlblock 502 may perform any desired procedure to determine how to adjustthe digital control word (B_(C)) 404 to coarsely tune the outputfrequency. Potential procedures include non-linear control algorithmsand linear control algorithms. For example, a non-linear controlalgorithm could be implemented in which a simple “too fast” or “tooslow” frequency comparison determination is made between the dividedoutput frequency (f_(OUT)/N) 218 and the divided reference frequency(f_(REF)/R) 216, and the digital control block 502 may use a successiveapproximation algorithm to coarsely tune the output frequency (f_(OUT))102. Alternatively, a linear control algorithm could be implemented inwhich a quantitative frequency comparison determination is made aboutthe approximate size of the frequency error between the divided outputfrequency (f_(OUT)/N) 218 and the divided reference frequency(f_(REF)/R) 216, and the control block 812 may change the digitalcontrol word (B_(C)) 404 by an appropriate amount to compensate for thesize of the frequency error. It is noted that the procedure used maydepend upon numerous variables including the particular applicationinvolved and the level of coarse tuning desired.

FIG. 8 is a block diagram of an embodiment for discrete controlcircuitry 502 according to the present invention for providing thedigital control word (B_(C)) 404 to the discretely variable capacitance(C_(D)) 402. The embodiment of FIG. 8 may be utilized in conjunctionwith the procedure set forth in TABLE 2 below and the number andweightings for the capacitances as set forth in TABLE 1. As indicated inFIG. 8, the digital control word (B_(C)) 404 may be an N+1 bit signal.The number of bits selected for “N+1” depends upon the number of controlsignals (0:N) desired to be sent to the discretely variable capacitance(C_(D)) 402. It is also noted that the “N” used with respect to thedigital control word (B_(C)) 404 is not the same as the “N” used withrespect to the divide-by-N (÷N) counter 214.

For the embodiment for discrete control block 502 of FIG. 8, and unlikethe embodiment depicted in FIG. 5, the divided reference frequencysignal (f_(REF)/R) 216 is not connected to the discrete controlcircuitry 502. Rather, the reference frequency (f_(REF)) 106 is directlyconnected to the control block 812. The control block 812 uses thereference frequency signal (f_(REF)) 106 to define a reference clock(T_(REF)) that is equivalent to the time period (T_(REF)=1/f_(REF)) foreach cycle of the reference frequency signal (f_(REF)) 106. The controlblock 812 sends to a frequency comparison block 804 a timing signal(R·T_(REF)) 814 that has a cycle time of R times the unit cycle time(T_(REF)). The frequency comparison block 804 also receives thedivided-by-N output frequency signal (f_(OUT)/N) 218.

It is noted that the timing signal (R·T_(REF)) 814 may also be used togenerate a divided reference frequency signal that is equivalent to thedivided-by-R reference frequency signal (f_(REF)/R) 216 generated by thedivide-by-R (÷R) counter 204 of FIG. 5. To do so, the timing signal(R·T_(REF)) 814 may be sent, as indicated in FIG. 8, to clock aflip-flop circuit that receives the reference frequency signal (f_(REF))106. The flip-flop circuit may thereby generate a divided-by-R referencefrequency signal (f_(REF)/R) 216 that is relatively jitter-free. Thisresulting divided reference frequency signal (f_(REF)/R) 216 may be usedwithin the PLL circuitry depicted in FIG. 5.

To initiate a frequency comparison, the control block 812 may firstsynchronize the divided output frequency signal (f_(OUT)/N) 218 with thetiming signal (R·T_(REF)) by resetting the divide-by-N (÷N) counter 214through the assertion of a reset signal (RESET) 816 that may be appliedto the divide-by-N (÷N) counter 214. The control block 812 may thenassert the timing signal (R·T_(REF)) 814 for a single cycle. At the endof this single cycle for the timing signal (R·T_(REF)) 814, thefrequency comparison block 804 may determine whether or not the dividedoutput frequency (f_(OUT)/N) 218 is “too fast” or “too slow” withrespect to the timing signal (R·T_(REF)) 814 and may provide somequantification of the amount of the frequency error, if so desired. Thefrequency comparison block 804 provides back to the control block 812 asignal 811 that is indicative of the frequency comparison determinationmade and that may be a single bit signal or a multiple bit signal, asdesired. The control block 812 may then adjust the digital control word(B_(C)) 404 accordingly to coarsely tune the output frequency (f_(OUT)).

When the control block 812 completes its coarse tuning procedure, thecontrol block 812 may assert the START signal 506 to change switch 512to deselect voltage control node 510 and pass control to the voltagecontrol node 508 (as shown in FIG. 5). At this point, the existingdigital control word (B_(C)) 404 may be fixed such that the discretelyvariable capacitance (C_(D)) 402 will remain the same, while thecontinuously variable capacitance (C_(A)) 406 is varied. In this way,the integrated circuit may operate to initially calibrate the outputfrequency (f_(OUT)) 102 to a desired output frequency, such that acoarse level of tuning control is provided by the discretely variablecapacitance (C_(D)) 402 and a fine level of tuning control is providedby the continuously variable capacitance (C_(A)) 406.

In TABLE 2 below, an example procedure for control block 812 isdescribed in more detail for controlling the digital control word(B_(C)) 404. This procedure correlates to the capacitor weighting schemeset forth in TABLE 1 in which the number of capacitor/switch circuitswas selected to be eleven. The number of bits in the digital controlword (B_(C)) 404 has also been chosen to be eleven. It is noted that theprocedure implemented will depend upon design considerations and thatany desired procedure may be implemented. Like TABLE 1, TABLE 2 belowwas contemplated for a dual band cellular phone application as depictedand described with respect to FIGS. 6A and 6B above. As discussed above,capacitors are added by changing their respective control bit to a “1”and are dropped by changing their respective control to a “0”. (If PMOStransistor switch circuits were utilized instead of NMOS transistorswitch circuits, these control bits would of course change accordinglyso that a “0” would add in the capacitor and a “1” would drop thecapacitor.)

TABLE 2 Example Procedure for Choosing Capacitor Values with a DigitalControl Word PROCEDURE OPERATIONS PERFORMED Cycle 13 Set B_(c) [10:0] =11001000000, i.e., switch in C[10], C[9], and C[6]. VCO Warm-up. Cycle12 Reset the N divider to start frequency comparison. Cycle 11 Dofrequency comparison. If VCO too slow, drop C[9] and keep C[10] andC[6]. Otherwise, if too fast, keep C[10] and C[9], drop C[6], and switchin C[8] and C[5]. Also, directly go to Cycle 8 at the end of Cycle 11.Reset the N divider to start frequency comparison. Cycle 10 Do frequencycomparison. If VCO too slow, drop C[10]. Otherwise, if too fast, keepC[10]. Add C[9]. Reset the N divider to start frequency comparison.Cycle 9 Do frequency comparison. If too slow, drop C[9] and C[6].Otherwise, keep C[9], drop C[6]. Add C[8] and C[5]. Reset the N dividerto start frequency comparison. Cycle 8 Do frequency comparison. If tooslow, drop C[8] and C[5]. Otherwise, keep C[8], drop C[5]. Add C[7] andC[4]. Reset the N divider to start frequency comparison. Cycle 7 Dofrequency comparison. If too slow, drop C[7] and C[4]. Otherwise, keepC[7], drop C[4]. Add C[6] and C[3]. Reset the N divider to startfrequency comparison. Cycle 6 Do frequency comparison. If too slow, dropC[6] and C[3]. Otherwise, keep, C[6], drop C[3]. Add C[5] and C[2].Reset the N divider to start frequency comparison. Cycle 5 Do frequencycomparison. If too slow, drop C[5] and C[2]. Otherwise, keep C[5], dropC[2]. Add C[4] and C[1]. Reset the N divider to start frequencycomparison. Cycle 4 Do frequency comparison. If too slow, drop C[4] andC[1]. Otherwise, keep C[4], drop C[1]. Add C[3] and C[0]. Reset the Ndivider to start frequency comparison. Cycle 3 Do frequency comparison.If too slow, drop C[3] and C[0]. Otherwise, keep C[3], drop C[0]. AddC[2]. Reset the N divider to start frequency comparison. Cycle 2 Dofrequency comparison. If too slow, drop C[2]. Otherwise, keep C[2]. AddC[1]. Reset the N divider to start frequency comparison. Cycle 1 Dofrequency comparison. If too slow, drop C[1]. Otherwise, keep C[1]. AddC[0]. Reset the N divider to start frequency comparison. Cycle 0 Dofrequency comparison. If too slow, drop C[0]. Otherwise, keep C[0].Reset the N divider to start the normal operation.

It is noted that the procedure in TABLE 2 is a non-linear controlalgorithm that implements a type of successive approximation (SA)scheme. The procedure has been designed to take thirteen steps tocomplete. This scheme uses the frequency comparison block 804 in FIG. 8to make a “too fast” or “too slow” determination for the divided outputfrequency (f_(OUT)/N) 218 with respect to the divided referencefrequency (f_(REF)/R) 216. One method for making this frequencycomparison is to first reset the N divider 214 to synchronize it for thefrequency determination and then to latch the level of the dividedoutput frequency (f_(OUT)/N) 218 R number of reference clocks (T_(REF))later as controlled by the timing signal (R·TREF) 814 from the controlblock 812. A number of additional reference clocks (T_(REF)) may then beutilized to allow time for other actions, which may include for exampletime for the control block 812 to keep or drop capacitance values byupdating the digital control word (B_(C)) 404 in response to thecomparison signal 811 and time for the control block 812 to reset thedivide-by-N (÷N) counter 214 for the next cycle in TABLE 2. For theprocedure set forth in TABLE 2, it is noted for example that the valuefor R may be thirty-nine, that the additional reference clocks (T_(REF))may be two, and that each cycle in TABLE 2 therefore lasts forty-onereference clocks (T_(REF)). It is further noted that the procedure ofTABLE 2 may be implemented by programming the desired procedure usingthe VERILOG logic circuit programming language and thereaftersynthesizing the desired circuitry.

For each frequency comparison, if the level is high (logic level “1”),the comparison signal 811 to control block 812 that the divided outputfrequency (f_(OUT)/N) 218 is too fast. If the level is low (logic level“0”), the comparison signal 811 indicates to control block 812 that thedivided output frequency (f_(OUT)/N) 218 is too slow. To synchronize thefrequency comparison determination, the divide-by-N (÷N) counter 214 maybe reset with respect to the timing signal (R·T_(REF)) 814 so that thedivided output frequency (f_(OUT)/N) 218 starts at the initiation of thetiming signal (R·T_(REF)) 814. This reset synchronization is indicatedin the cycles described in TABLE 2. It is noted that a “too fast” or“too slow” determination may also be made by directly comparing thereference frequency (f_(REF)) to the output frequency (f_(OUT)), if sodesired.

As a general rule, the procedure in TABLE 2 operates by starting with alarge capacitance value and in each cycle either dropping capacitancevalues, if the divided output frequency (f_(OUT)/N) 218 is too slow, orkeeping capacitance values, if the divided output frequency (f_(OUT)/N)218 is too high. In this manner, each successive cycle keeps or dropsvarious capacitance values until the end of the procedure is reached. Asdepicted in Cycle 13 of TABLE 2, more capacitance than just the largestcapacitance value (C[10]) may be initially included, if desired, to slowdown the initial output frequency when the actual frequency is generallymost uncertain.

Within this procedure as described in TABLE 2, a level of redundancy isalso implemented that allows recovery from bad comparisons or decisionsmade by frequency comparison block 804 or the control block 812. Forsuccessive approximation type algorithms, it is typically easier torecover from erroneously dropping capacitance values, while it istypically very difficult to recover from erroneously keeping capacitancevalues. In addition, manufacturing tolerances may create significantproblems because the capacitance values are not what they are desired tobe. To compensate for these recovery and tolerance problems, thecapacitance values may be purposefully manufactured in the radixless-than-two scheme described above. In this way, errors will tend tobe “drop” errors. To further improve redundancy and error recovery, thevalues chosen for the capacitor weightings and the number of acapacitors utilized may be chosen such that a degree of value overlap isachieved.

It is again noted that the procedure of TABLE 2 and the capacitor valuesmay be modified as desired and that numerous alternative circuit designsmay be utilized, while still achieving a discretely variable capacitancecircuit as contemplated by the present invention.

Further modifications to the circuit implementation of FIG. 7 will nowbe discussed with respect to FIGS. 9A, 9B and 9C.

FIG. 9A depicts a circuit representation of the capacitor (C_(Di)) 902and NMOS transistor switch (S_(i)) 908 circuits in FIG. 7 when thecontrol signal (B_(i)) 910 is at a low or “off” logic level. Thecapacitor (C_(Di)) 902 will have a parasitic capacitance (C_(Dipar)) 907between its bottom plate and ground 412. In this “off” state, the NMOStransistor (S_(i)) 908 may be represented as a diode (D_(i)) 906, whichis desirably reverse-biased, and a parasitic capacitance (C_(Sipar)) 904in parallel with a switch (S_(i)) 908. The switch (S_(i)) 908 has itscontrol node 910 set to an “off” state (B_(i)=0). The parasiticcapacitance (C_(Sipar)) 904 will provide an undesirable non-linearcapacitance value proportional to the size (width/length=W/L) of thetransistor. Although not shown, the NMOS transistor (S_(i)) 908 may berepresented by some resistance (R_(DS)) in series with the capacitor(C_(Di)) 902, when it is in its “on” state.

Potential problems with this capacitor/switch circuit arise. In the“off” state, problems include the capacitor parasitic capacitance(C_(Dipar)) 907, the transistor parasitic capacitance (C_(Sipar)) 904,and possible forward biasing of the diode (D_(i)) 906. In its “on”state, problems include phase noise contribution that is dependent uponthe size of the series resistance (R_(DS)). Because the capacitorparasitic capacitance (C_(Dipar)) 907 will be linear, it is typicallynot considered much of a problem, although it limits the switchablecapacitance range. In contrast, the transistor parasitic capacitance(C_(Sipar)) 904 will be non-linear and will convert any voltage noise onnode 917 to phase noise. When the NMOS transistor (S_(i)) 908 is in its“off” state, therefore, it is desirable for the transistor parasiticcapacitance (C_(Sipar)) 904 to be as low as possible. This desired smallcapacitance translates into selecting a small size for the NMOStransistor (S_(i)) 908. When the NMOS transistor (S_(i)) 908 is in its“on” state, however, it is desirable for the series resistance (R_(DS))to be as low as possible. This desired low resistance may be achieved byselecting a large size for the NMOS transistor (S_(i)) 908. The size ofthe NMOS transistor (S_(i)) 908, therefore, affects the transistorparasitic capacitance (C_(Sipar)) 904 and the series resistance (R_(DS))in an inverse relationship. Thus, a balancing decision must be made inselecting the transistor size.

If all of the NMOS transistors 710, 712, 714 and 716 (S₀, S₁ . . .S_(N−1), S_(N)) are sized the same, the parasitic capacitances, as wellas the capacitance drift, for each will be approximately the same. Theselected capacitance value for each successively smaller capacitor(C_(Di)) 902 will become closer to the capacitance value of theparasitic capacitance (C_(Sipar)) 904. For smaller capacitance values,the parasitic capacitance drift begins to adversely affect capacitormatching within the array. One way to make the balancing decisionmentioned above, therefore, is to decide upon a particular value foreach transistor parasitic capacitance (C_(Sipar)) 904 as a percentage ofthe value for each capacitor (C_(Di)) 902. In this way, the transistorparasitic capacitances will be scaled in proportion to the scaling ofthe overall capacitances.

FIG. 9B provides an embodiment for such a solution. This embodimentcontemplates reducing the size of each successive transistor (S_(i)) 908in proportion with the reduction in value of each successive capacitor(C_(Di)) 902, according to the weighting scheme adopted. As depicted inFIG. 9B, the scaling factor between two adjacent capacitor values isshown as 1/k. In other words, the value of a first capacitor (C_(D0))702 would be 1/k times the value of a second capacitor (C_(D1)) 704. Toreduce drift problems and improve capacitor matching, the size (W/k) ofthe each transistor (S_(i)) is made to be 1/k times the size (W) of thecorresponding next larger transistor.

Because of semiconductor manufacturing process limitations, the width ofthe smallest transistor elements may be limited to a particular value.At some point, therefore, the size will no longer be able to beproportionally reduced with respect to the previous transistor sizevalue. FIG. 9C provides a circuit that solves this problem by adding afixed capacitor across the drain and source of the transistor (S_(i))908. Because of the size limitation mentioned above, the size (W/L) ofthe transistor (S_(i)) 908 is the same as the previous transistor withinthe array. Instead of reducing the size of the transistor (S_(i)) 908, acapacitor 912 having a value of (k−1)/k times the value of the previouscapacitor value is added. The result is an overall switchablecapacitance value that is equivalent to the circuit depicted in FIG. 9Bwithout reducing the size of the transistor 908.

FIG. 9C also depicts an embodiment for avoiding the problem withpossible forward biasing of the diode (D_(i)) 906 when the NMOStransistor (S_(i)) 908 is in its “off” state. The PMOS transistor 916,which has its drain and source terminals connected between signal line414 and node 917, avoids this “off” state diode problem by keeping thevoltage at node 917 from floating when transistor 908 is in its “off”state. This PMOS transistor 916 is controlled by a signal (B_(i) _(—)hat) 918 that is identical to the control signal (B_(i)) 910, exceptthat the “_hat” designation represents that the signal (B_(i) _(—) hat)918 has a regulated voltage when high. In other words, signal (B_(i)_(—) hat) 918 may only rise to a predetermined voltage level. Thus, whenthe bit (B_(i)) 910 goes to a high logic level, the signal (B_(i) _(—)hat) 916 will only go to the regulated high voltage level. The additionof PMOS transistor 916 connects what would otherwise be an uncontrolledand potentially noisy floating node to a well-determined and quietsignal line 414. In this way, PMOS transistor 916 tends to eliminate thepotential problem of the diode (D_(i)) 906 being forward biased due to afloating voltage at node 917.

Still considering the embodiment depicted in FIG. 7, a differentialimplementation will now be discussed with respect to FIG. 10, FIG. 11and FIG. 12.

FIG. 10 is a block diagram of a differential embodiment for a VCO 400according to the present invention. Compared to the embodiment for VCO400 depicted in FIG. 4, the circuit elements are essentially duplicatedfor a positive and negative input paths. A differential amplifier 1002is connected to a negative output node 414N and a positive output node414P, as well as to ground 412. An external inductor (L_(EXT)) 302 maybe connected across the output nodes 414P and 414N. A positive sidefixed capacitor (C_(FP)) 410P, a discretely variable capacitance(C_(DP)) 402P, and continuously variable capacitance (C_(AP)) 406P maybe connected between the positive output node 414P and ground 412.Similarly, a negative side fixed capacitor (C_(FN)) 410N, a discretelyvariable capacitance (C_(DN)) 402N, and continuously variablecapacitance (C_(AN)) 406N may be connected between the negative outputnode 414N and ground 412. The voltage control signal (V_(C)) 408 may bean M+1 bit signal, as further described below, and may be applied toboth the positive and negative side continuously variable capacitances(C_(AP), C_(AN)) 406P and 406N. The digital control word (B_(C)) 404 maybe an N+1 bit signal, as described above, and may be applied to both thepositive and negative side discretely variable capacitances (C_(DP),C_(DN)) 402P and 402N.

FIG. 11 is a circuit diagram of a differential amplifier 1002 accordingto the present invention. A first PMOS transistor 1104 has its sourceconnected to a voltage supply control node (V_(SRC)) 1102 and its drainconnected to the positive output node 414P. The gate of the first PMOStransistor 1104 may be connected to the negative output node 414N.Similarly, a second PMOS transistor 1106 has its source connected to avoltage supply control node (V_(SRC)) 1102 and its drain connected tothe negative output node 414N. The gate of the second PMOS transistor1106 may be connected to the positive output node 414P. A first NMOStransistor 1108 has its source and drain connected between ground 412and the positive output node 414P. The gate of the first NMOS transistor1108 may be connected to the negative output node 414N. Similarly, asecond NMOS transistor 1110 has its source and drain connected betweenground 412 and the negative output node 414N. The gate of the secondNMOS transistor 1110 may be connected to the positive output node 414P.The voltage supply control node (V_(SRC)) 1102 may be used to controlthe level of the output signal. The output amplitude may be monitored.If the amplitude is too high, the voltage on the voltage supply controlnode (V_(SRC)) 1102 may be lowered. Conversely, if the amplitude is toolow, the voltage on the voltage supply control node (V_(SRC)) 1102 maybe raised. It is noted that the voltage supply control node (V_(SRC))1102 may be supplied by voltage or current in open loop or closed loopto provide the desired control.

FIG. 12 is a circuit diagram depicting transistors that may be added toimprove the performance of each capacitor pair within the discretelyvariable capacitance (C_(D)) 402. Positive side capacitor (C_(DPi)) 902Pand transistor (Spi) 908P and negative side capacitor (C_(DNi)) 902N andtransistor (S_(Ni)) 908N form differential capacitor/switch pairs forthe discretely variable capacitance (C_(D)) 402. The transistors(S_(Pi)) 908P and (S_(Ni)) 908N are controlled by the same controlsignal (B_(i)) 910. As described with respect to FIG. 9C above, the PMOStransistors 916P and 916N may be added to avoid an “off” state diodeproblem by keeping the voltages at nodes 917P and 917N from floatingwhen switches 908P and 908N are in an “off” state. These PMOStransistors 916P and 916N are controlled by the regulated signal(B_(i—)hat) 918.

As discussed above with respect to FIGS. 9A, 9B and 9C, the seriesresistance (R_(DS)) and the transistor parasitic capacitance (C_(Sipar))are in a trade-off relationship dependent upon the size of thetransistor. Without the NMOS transistor (S_(PNi)) 1210, the transistors(S_(Pi)) 908P and (S_(Ni)) 908N would have a total series resistance(R_(D)) of a certain amount when they are in their “on” state. Thisamount may be significantly reduced by the addition of NMOS transistor(S_(PNi)) 1210. For example, by choosing a size value for transistor(S_(PNi)) 1210 of about three times the size of transistors (S_(Pi))908P and (S_(Ni)) 908N, individually, the total effective seriesresistance (R_(DS)) of the transistors (S_(Pi)) 908P, (S_(Ni)) 908N, andtransistor (S_(PNi)) 1210 may be effectively reduced by nearly half fromthe original amount. Thus, the NMOS transistor (S_(PNi)) 1210 allows thebalancing decision to be made further in favor of reducing the size ofthe transistor parasitic capacitance (C_(Sipar)) when the transistors(S_(Pi)) 908P and (S_(Ni)) 908N are in an “off” state. The NMOStransistor (S_(PNi)) 1210 is controlled by the same control signal(B_(i)) 910 in the digital control word 404 that controls thetransistors (S_(Pi)) 908P and (S_(Ni)) 908N.

It is noted that three separate differential VCOs 400 implemented asdepicted in FIGS. 10-12 may be utilized to synthesize the RF1, RF2, andIF output frequencies, as described above with respect to FIGS. 6A, 6B,13 and 14. Example component values will now be described for theexternal inductor (L_(EXT)) 302, the fixed capacitances (C_(FP), C_(FN))410P and 410N, the total capacitance for the discretely variablecapacitances (C_(DP), C_(DN)) 402P and 402N, and the total transistorsize for transistor switches (S_(Pi), S_(Ni), S_(PNi)) 908P, 908N and1210 within the discretely variable capacitances (C_(DP), C_(DN)) 402Pand 402N. These example values assume the number of capacitors and theweighting scheme described with respect to TABLE 1 and the coarse tuningprocedure described with respect to TABLE 2 are being utilized.

For an RF1 output frequency with a maximum center frequency of about 2.0GHz, the external inductor (L_(EXT)) may be about 2.0 nH. The two fixedcapacitances (C_(FP), C_(FN)) 410P and 410 N may each be about 2.0 pF.The summation of the eleven capacitance values within the positive sidediscretely variable capacitance (C_(DP)) 402P and the eleven capacitancevalues within the negative side discretely variable capacitance (C_(DN))402N may each be about 7.5 pF. Each of these eleven capacitance valuesare weighted as indicated in TABLE 1 with the unit weight (C₀) beingequal to the total capacitance of 7.5 pF divided by the total of theweightings, which as set forth in TABLE 1 is 677. The summation of thewidths for each eleven groupings of the transistor switches (S_(Pi),S_(Ni), S_(PNi)) 908P, 908N and 1210 may be 1280 μm, 1280 μm, and 3840μm, respectively. Each of the eleven transistor width values areweighted according to the weights given their respective capacitors(C_(DPi), C_(DNi)) 902P and 902N in TABLE 1 with the unit transistorwidth being equal to the total width of 1280 μm or 3840 μm divided bythe total of the weightings of 677. (It is noted that the techniquedepicted in FIG. 9C is utilized for any of the smallest transistorwidths that fall below the minimum width allowed by the semiconductormanufacturing process utilized.) The transistor lengths are not variedand are all 0.35 μm. The total impedance for all of the transistorswitches (S_(Pi), S_(Ni), S_(PNi)) 908P, 908N and 1210 is about 0.56Ωfor worst case conditions.

For an RF2 output frequency with a maximum center frequency of about 1.3GHz, the external inductor (L_(EXT)) may be about 3.1 nH. The two fixedcapacitances (C_(FP), C_(FN)) 410P and 410N may each be about 3.6 pF.The summation of the eleven capacitance values within the positive sidediscretely variable capacitance (C_(DP)) 402P and the eleven capacitancevalues within the negative side discretely variable capacitance (C_(DN))402N may each be about 11.1 pF. Each of these eleven capacitance valuesare weighted as indicated in TABLE 1 with the unit weight (C₀) beingequal to the total capacitance of 11.1 pF divided by the total of theweightings, which as set forth in TABLE 1 is 677. The summation of thewidths for each eleven groupings of the transistor switches (S_(Pi),S_(Ni), S_(PNi)) 908P, 908N and 1210 may be 1568 μm, 1568 μm, and 4704μm, respectively. Each of the eleven transistor width values areweighted according to the weights given their respective capacitors(C_(DPi), C_(DNi)) 902P and 902N in TABLE 1 with the unit transistorwidth being equal to the total width of 1568 μm or 4704 μm divided bythe total of the weightings of 677. (It is noted that the techniquedepicted in FIG. 9C is utilized for any of the smallest transistorwidths that fall below the minimum width allowed by the semiconductormanufacturing process utilized.) The transistor lengths are not variedand are all 0.35 μm. The total impedance for all of the transistorswitches (S_(Pi), S_(Ni), S_(PNi)) 908P, 908N and 1210 is about 0.46Ωfor worst case conditions.

For an IF output frequency with a maximum center frequency of about 600MHz, the external inductor (L_(EXT)) may be about 6.7 nH. The two fixedcapacitances (C_(FP), C_(FN)) 410P and 410N may each be about 8.0 pF.The summation of the eleven capacitance values within the positive sidediscretely variable capacitance (C_(DP)) 402P and the eleven capacitancevalues within the negative side discretely variable capacitance (C_(DN))402N may each be about 23.8 pF. Each of these eleven capacitance valuesare weighted as indicated in TABLE 1 with the unit weight (C₀) beingequal to the total capacitance of 23.8 pF divided by the total of theweightings, which as set forth in TABLE 1 is 677. The summation of thewidths for each eleven groupings of the transistor switches (S_(Pi),S_(Ni), S_(PNi)) 908P, 908N and 1210 may be 3200 μm, 3200 μm, and 9600μm, respectively. Each of the eleven transistor values are weightedaccording to the weights given their respective capacitors (C_(DPi),C_(DNi)) 902P and 902N in TABLE 1 with the unit transistor width beingequal to the total width of 3200 μm or 9600 μm divided by the total ofthe weightings of 677. (It is noted that the technique depicted in FIG.9C is utilized for any of the smallest transistor widths that fall belowthe minimum width allowed by the semiconductor manufacturing processutilized.) The transistor lengths are not varied and are all 0.35 μm.The total impedance for all of the transistor switches (S_(Pi), S_(Ni),S_(PNi)) 908P, 908N and 1210 is about 0.22Ω for worst case conditions.

The techniques discussed above have been shown with reference to afrequency synthesizer in which the fine tuning analog control isaccomplished with standard PLL components. For example with reference toFIG. 5, a phase detector 206, a charge pump 208, and a loop filter 210may be used to provide the voltage control for a voltage controlledoscillator. However, in order to more easily integrate the PLL within asingle integrated circuit, an alternative PLL design may be utilized.For example, as shown in FIG. 15, a PLL 1500 may be formed in which thephase detector 206, charge pump 208, and loop filter 210 are replacedwith a shift register 1504 and a phase detector/sample hold circuit 1502which has M+1 outputs 1512 which may be switched to the M+1 controlinputs 1514 of the VCO 400. The PLL 1500 provides the digital coursetuning under control of the discrete control circuit 502 as describedabove. The fine tuning analog control is provided through the use of thedivide-by-R block 204, divide-by-N block 214, divide-by-Q block 1550,the shift register 1504 and the phase detector/sample hold circuit 1502.

FIG. 16 illustrates the PLL 1500 of FIG. 15 during the fine tuning modeof operation (i.e. after coarse tuning is completed). In FIG. 16, onlythe analog control portions of the PLL are shown. As shown in FIG. 16, aVCO 400 (such as, for example, VCO 400 of FIG. 4) provides an outputfrequency (f_(out)) 102. As shown in FIG. 16, the VCO input controlsignals 408 of FIG. 4 have been replaced by a M+1 voltage control inputs1514. The output 102 is provided through control line 1508 to thedivide-by-N circuit 214. An output 1510 of the divide-by-N circuit 214is provided to a shift register 1504. The shift register 1504 is clockedby the output of the divide-by-Q block 1550 as shown. Parallel outputs1520[0, 1, 2, . . . M−1, M] of the shift register 1504 are provided tothe phase detector/sample hold circuit 1502. The output 1530 of thedivide-by-R circuit 204 is also provided to the phase detector. As willbe described in more detail below, each of the outputs 1520 of the shiftregister 1504 has the same frequency (f_(OUT)/N), i.e. the update rateof the PLL, but is incrementally shifted in phase from each other. Thephase detector/sample hold circuit 1502 detects the phase differencesbetween the output 1530 of the divide-by-R circuit 204 and each of theoutputs 1520[0, 1, 2, . . . M] of the shift register 1504. Outputs1512[0, 1, 2, . . . M] of the phase detector/sample hold circuit 1502provide control voltages indicative of the detected phase differences.These control voltages are in turn coupled to the inputs 1514[0, 1, 2, .. . M−1, M] of the VCO 400 through which fine tuning control of the VCOmay be achieved (as described below). Thus, rather than the VCO 400being provided a single analog control voltage, a plurality of controlvoltages may be provided. As discussed below, in one embodiment twentyoutputs may be provided from the shift register to generate twentyoutputs of the phase detector/sample hold circuit 1502 and twenty inputsto the VCO. Because the shift register 1504 produces a series ofoutputs, each shifted in phase, the control voltages provided to the VCOwill be a series of voltages offset from each other.

The use of multiple analog inputs to perform the fine control of the VCOmay be seen with reference to FIGS. 4 and 15-18. As discussed above withreference to FIG. 4, the fine analog control of the VCO 400 may beachieved through the use of the continuously variable capacitance(C_(A)) 406. As shown in FIG. 4, the continuously variable capacitance(C_(A)) 406 is controlled by the voltage control signal (V_(C)) 408. Itwill be recognized that the circuitry of FIG. 4 may be implemented withthe PLL 1500 of FIGS. 15 and 16 by using the plurality of voltagecontrol inputs 1514[0, 1, 2, 3, . . . M−1, M] to replace the voltagecontrol signal (V_(C)) 408.

FIG. 17A illustrates one exemplary embodiment of a variable capacitancecircuit 1700 which includes capacitors C1 and C2 and a variableimpedance device R_(V). The equivalent capacitance seen at the inputs tothe circuit 1700 will change depending upon the value of the variableimpedance device R_(V) that is coupled to a control node 1705. FIG. 17Billustrates the variable capacitance circuit 1700 wherein the variableimpedance device is a transistor 1702 having a gate controlled by ananalog voltage source. A plurality of the devices of FIG. 17B may beutilized to provide the variable capacitance (C_(A)) 406 of FIG. 4 undercontrol of the plurality of voltage control inputs 1514[0, 1, 2, 3, . .. M−1, M] of FIG. 16. For example, FIG. 17C illustrates how the variablecapacitance (C_(A)) 406 may be comprised of a plurality of variablecapacitance circuits C_(A0), C_(A1), . . . C_(AM) such thatC_(A)=C_(A0)+C_(A1), + . . . +C_(AM). Each capacitance circuit has avariable equivalent capacitance respectively controlled by the controlvoltage 1514[0], 1514[1], . . . or 1514[M]. The transistors T₀, T₁, . .. T_(M) act as variable control elements having a variable resistance inresponse to the analog control voltages. Each variable capacitancecircuits C_(A0), C_(A1), . . . C_(AM) includes a resistor (R₀, R₁, . . .or R_(M) respectively) not shown in the circuit 1700 of FIGS. 17A and 17B. Each resistor R₀, R₁, . . . R_(M) is provided to prevent the nodebetween the capacitors C1 and C2 from floating when the respectivetransistor T₀, T₁, . . . or T_(M) is fully turned off. The resistors R₀,R₁, . . . R_(M) may be formed from transistors sized to provide aresistance that is approximately 40 times the impedance of the sum ofthe capacitors C1 and C2 at oscillation frequencies.

The operation of an individual variable capacitance circuit will bedescribed with reference to capacitance circuit C_(A0). As shown in FIG.17C, capacitance circuit C_(A0) includes capacitors C1 ₀ and C2 ₀,resistor R₀ and the n-channel transistor T₀. The complex admittance seenacross the capacitance circuit C_(A0) may be characterized asY_(A0)(jω)=jωC_(equivalent)+G_(equivalent). Further, assuming theresistance value large compared to the impedance of the capacitors, asthe control voltage 1514[0] rises, the transistor will turn on morefully and the capacitance C_(equivalent) will approach C1 ₀. Similarly,as the control voltage 1514[0] approaches zero, the transistor will turnoff and the capacitance for C_(equivalent) will approach the value of(C1 ₀)(C2 ₀)/(C1 ₀+C2 ₀). As analog control voltages between the twoextremes are generated, the capacitance C_(equivalent) will pass throughall the capacitance points between the two values given above. Theconductance G_(equivalent) of the circuit C_(A0) will also varydepending upon the control voltages. At both extremes of the analogcontrol voltages, G_(equivalent) will approach zero. FIG. 17Dillustrates ωC_(equivalent) vs. G_(equivalent) as the control voltage ontransitor T₀ goes from a level to fully turn on the transistor to alevel to fully turn off the transistor. As can be seen, the locus ofpoints forms a semi-circle.

Thus, as the voltage level of an individual control voltage 1514[0]increases the capacitance C_(A0) will increase which thus increases thecapacitance C_(A). An increase in the capacitance C_(A) in turn lowersthe output frequency of the VCO 400. In this manner, higher controlvoltages result in lower VCO output frequencies.

As noted above with reference to FIG. 17C, a plurality of thecapacitance circuits (C_(A0), C_(A1), . . . C_(AM)) may be utilized andthe continuously variable capacitance C_(A) 406 is a result of thesummation of the individual capacitance circuits. The overallconductance (G_(equivalent) for C_(A)) of all of the capacitancecircuits operating together, however, does not increase beyond theG_(equivalent). FIG. 18 demonstrates this concept. FIG. 18 demonstratesa C_(equivalent) vs. G_(equivalent) over for the continuously variablecapacitance C_(A) 406. As shown in FIG. 18 for illustrative purposes,the additive effect of incrementally fully turning on each transistorT₀, T₁, . . . T_(M) is displayed. As can be seen, the total capacitancerange for C_(A) (ΔC_(A)) (the sum of the ranges of the individualcapacitance circuits) may be relatively large, without a correspondinglarge change in conductance. This characteristic of the circuit of FIG.17C helps minimize phase noise since the phase noise for the circuit isproportional to G_(equivalent). Thus, a wide capacitance range for thecontinuously variable capacitance C_(A) 406 is provided without causingexcessive phase noise. It may also be noted that as the control voltageon each control line 1514[0, 1, . . . or M] changes from rail to rail,only a fraction of the total capacitance range is changed (i.e. whereM+1=20 only {fraction (1/20)}^(th) of the total capacitance range).Thus, noise on any one particular control line will only have a minimalimpact on the total capacitance C_(A).

As seen in FIG. 18, the capacitance for each individual capacitancecircuit is shown not to overlap an adjacent circuit for illustrativepurposes. However, in operation the values chosen for C1 and C2, thenominal frequency of oscillation, and the size of the transistor foreach circuit and the time at which each circuit is activated may be suchthat the individual portions of the graph of FIG. 18 may overlap.

The use of multiple control voltages to provide the continuous analogfine control of the VCO may be accomplished in a wide range of manners,and the embodiment shown herein is just one example. FIG. 19 illustratesyet another embodiment for implementing the continuously variablecapacitance C_(A) 406. As shown in FIG. 19, the continuously variablecapacitance C_(A) 406 may be comprised of two capacitors C1 and C2,resistor R, and a plurality of transistors T₀, T₁, . . . T_(M)controlled by the control voltages 1514[0], 1514[1] . . . 1514[M]respectively.

The capacitance value of the continuously variable capacitance C_(A) 406(and the associated output frequency of the VCO) of FIG. 16 will bedependent upon the voltages present upon the control voltage lines 1514.Further by utilizing the control techniques discussed below, arelatively linear continuously variable capacitance C_(A) 406 may beobtained even though each individual capacitance circuit C_(A0) ,C_(A1), . . . C_(AM) may exhibit non-linear behavior.

The generation of the control voltages is described below with referenceto FIGS. 16 and 20-15. As shown in FIG. 16 the output frequency signal102 is provided through feedback line 1508 to a divide-by-N circuit 214.The divide-by-N circuit 214 may be programmable based upon user provideddata stored in an N register as described above. The divide-by-N circuitdivides the frequency provided on line 1508 to a lower frequency signalprovided on line 1510. Any of a wide number of circuits may be utilizedto perform the division function. Because of the high frequenciesencountered at the frequency output 102, it may be advantageous toutilize standard prescaler techniques when implementing the divide-by-Ncircuit. Such techniques for high frequency signals are well known suchas shown, for example, in J. Craninckx and M. S. Steyaert, “A1.75-GHz/3-V Dual-Modulus Divide-by-128/129 Prescaler in 0.7-um CMOS,”IEEE J. Solid-State Circuits, vol. 31, pp. 890-897, July 1996; B. Chang,J. Park, and W. Kim, “A 1.2 GHz CMOS Dual-Modulus Prescaler Using NewDynamic D-Type Flip-Flops,” IEEE J. Solid-State Circuits, vol 31, pp.749-752, May 1996; and H. -IH Cong, J. M. Andrews D. M. Boulin, S. -C.Fang, S. J. Hillenius, and J. A. Michejda, “Mutigigahertz CMOSDual-Modulus Prescaler IC,” IEEE J. Solid-State Circuits, vol. 23, pp.1189-1194, October 1988.

The output line 1510 of the divide-by-N circuit 214 is provided to ashift register 1504. The output 1510 is utilized to SET the multi-outputshift register 1504. In operation, when the SET signal is low on line1510, clocking of the shift register outputs is allowed and when the SETsignal is high the shift register outputs are set high. Thus, the shiftregister is set based upon the PLL update rate (f_(OUT)/N). The shiftregister 1504, however, is clocked at a higher frequency, such asf_(OUT)/Q as shown. Alternatively, the shift register 1504 may beclocked by f_(OUT) directly, however, the high frequencies of f_(OUT)may exceed the clocking speed limits of the shift register and thus thedivide-by-Q block 1550 may be desireable. Typical values for Q may beprogrammable, for instance to values of 8, 16, 32, and 64 (for RF1), 4,8, 16, or 32 (for RF2), and 2, 4, 8, or 16 (for IF). Thus, a signalpropagates through the shift register 1504 (and is provided at the shiftregister outputs) at the high f_(OUT)/Q frequency to provide a pluralityof signals, each at a frequency of f_(OUT)/N (the PLL update rate) buteach with falling edges out of phase with the others by increments ofthe Q/f_(OUT) period. Though the input of the divide-by-Q block 1550 isshown as being the fout signal for illustrative purposes, thedivide-by-Q input may be a sub-signal within the divide-by-N block 214utilized to obtain the desired f_(OUT)/Q result.

The generation of the shift register outputs may be seen with moredetail with reference to the shift register timing diagram shown in FIG.20. As shown in FIG. 20, the f_(OUT)/Q signal is provided to clock theshift register at a higher frequency than the frequency of the SETsignal (f_(OUT)/N). When the SET signal goes low, the shift register isclocked. Based upon edges of the f_(OUT)/Q signal, a low signalpropagates through the shift register outputs 1520[0], 1520[1], 1520[2],. . . 1520[M] as shown. When the SET signal returns high, the shiftregister outputs are set to their original high state. In this manner aseries of signals (the shift register outputs) are generated that havefalling edges each slightly out of phase from the adjacent signal, eachhaving a frequency at the PLL update rate f_(OUT)/N. This series ofsignals may then be provided to the phase detector/sample hold circuit1502. Though the f_(OUT)/Q signal is shown in FIG. 20 as continuouslyclocking the shift register, power usage may be decreased by onlyturning on the clock input for times between the falling edge of the SETsignal 1510 and the last falling edge of the shift register output(1520[M]).

The technique described herein to provide a plurality of signals for thephase detector/sample hold circuit 1502 is useful over a wide range ofapplications, including the generation of high frequency signals forwireless telephones. For example, in a typical PLL embodiment for usewith the GSM standard having 200 kHz channels, f_(OUT) may be 900 MHz, Qmay be 4, N may be 4500 to provide a SET signal at 200 kHz (i.e. theupdate period is 5000 nsec.). In this case, one output of the shiftregister changes approximately every 4.44 nsec., and if M+1=20, all theoutputs of the shift register will have changed states during the first88.8 nsec. after the SET signal goes low. Typically the value of Q maybe programmed so that (M+1)(Q/f_(OUT)), i.e. the maximum time requiredfor all outputs of the shift register to change, is approximately 2% ofthe entire update period (N/f_(OUT)).

The phase detector/sample hold circuit 1502 operates to compare thephases of the shift register outputs 1520[0], 1520[1], 1520[2], . . .1520[M] to the output 1530 (f_(REF)/R) of the divide-by-R circuit 204.Voltage outputs 1512[0], 1512[1], 1512[2], . . . 1512[M] of the phasedetector/sample hold circuit 1502 are provided at voltage levelsdependent upon the phase differences detected. A functional blockdiagram of a portion of the phase detector/sample hold circuit 1502 isshown in FIG. 21. FIG. 21 illustrates the function of a portion 1502[0]of the phase detector/sample hold circuit 1502 with reference to oneshift register output, for example, output 1520[0]. Thus, the phasedifference between shift register output 1520[0] and the divide-by-Routput 1530 is obtained in a phase detector 1502 A[0]. The phasedetector 1502 A[0] provides an output 2200 which is at a voltageV_(PHASE). The voltage level of V_(PHASE) is dependent upon the detectedphase difference. More particularly, V_(PHASE)=(k_(P))(Δθ) where k_(P)is a gain factor of the phase detector 1502A[0] and Δθ is the detectedphase difference. The output 2200 (V_(PHASE)) is then provided to asample and hold circuit 1502B[0] which generates the output 1512[0]. Theoutput 1512[0] may then be provided as a control voltage input to theVCO 400 as shown in FIG. 16.

A circuit 1502[0] for implementing the phase detector 1502A[0] andsample and hold circuit 1502B[0] of FIG. 21 is shown in FIG. 22. Asshown in FIG. 22, a voltage V_(NOM) is coupled to a capacitor C_(RAMP)by closing a charge switch 2302 (at this point calibration switches 2340and 2342 will be opened as discussed below with reference to calibrationtechniques). In operation, before a phase difference is to be detected,charge switch 2302 is closed to allow capacitor C_(RAMP) and theV_(PHASE) line 2200 to charge up to V_(NOM) (at this point sample/holdswitch 2304 is open). Then the charge switch 2302 is opened.Subsequently, charge begins to be removed from the capacitor C_(RAMP) byturning on one of the transistors 2320. However, the one transistor 2320that is turned on is only turned on for a time period indicative of thephase difference between the divide-by-R output signal 1530 and theshift register output 1520[0]. In this manner the voltage on theV_(PHASE) line 2200 will be related to the phase difference. FIG. 24Aillustrates the voltage levels for V_(PHASE). As seen in FIG. 24A, afterthe charge switch 2302 is opened V_(PHASE) is initially at V_(NOM). Thenwhen the divide-by-R output signal 1530 rises, V_(PHASE) begins to fall.When the shift register output 1520[0] also falls, the transistor 2320is turned off and V_(PHASE) is held constant. As shown in FIG. 24B, thefinal value of V_(PHASE) will vary as indicated by the dotted lines 2400depending on the phase difference between the divide-by-R output signal1530 and the shift register output 1520[0] (each dotted line indicativeof a different phase difference). In situations where the edge of thef_(OUT)/N clock leads the edge of the f_(REF)/R edge, the V_(PHASE)signal will not drop, and thus, V_(PHASE) will remain at V_(NOM).

One embodiment of a circuit for generating V_(NOM) is shown in FIG. 23.The voltage circuit 2360 includes a current source 2352, a resistor 2354and an amplifier 2356. The feedback loop through amplifier 2356 helpsimprove the noise characteristics of the amplifier. In operation, theswitch 2350 may be open to allow charge to be delivered in open loopconditions and then closed to also allow charge to be delivered inclosed loop conditions. Typically, a majority of the charge may bedelivered in open loop conditions so as to keep power supply currentconstant. In one embodiment, V_(NOM) may be a 1.9 V voltage source.

As mentioned above with reference to FIG. 22, only one transistor 2320is turned on at any given time. Multiple transistors 2320 are providedso that a selectable resistance between the V_(PHASE) line 2200 andground may be provided. In this manner the rate of decay of V_(PHASE).The rate of decay will impact the number of individual capacitancecircuits C_(A0) , C_(A1) . . . C_(AM) which are operating in theiractive range at any given time. The desired gain is controlled byselectively providing a high signal on one of the SEL1, SEL2, SEL3, orSEL4. In this manner, only one of the AND gates 2306, 2308, 2310, and2312 will provide a high output, and thus, only one of the transistors2320 will turn on and off in response to the rising and falling edges ofthe signals on lines 1530 and 1520[0].

The sample/hold function of the circuit 1502[0] is implemented throughuse of the sample/hold switch 2304 and the C_(HOLD) capacitor. After thephase difference has been detected as described above, the sample/holdswitch 2304 is closed. Thus, charge is now shared between the capacitorsC_(RAMP) and C_(HOLD) and the voltage level on output line 1512[0] willchange in response to the detected phase difference and on the voltageon C_(HOLD) before the switch is closed. After the voltage on output1512[0] provided to the VCO 400 has settled, sample/hold switch 2304 isopened again and the phase detection cycle may start again. Because ofthe charge sharing between the capacitors C_(RAMP) and C_(HOLD), thevoltage level on output 1512[0] after a current phase detection cyclewill depend on the charge on C_(HOLD) during the previous phasedetection cycle and the currently detected phase difference. Typicallythe capacitance ratio of C_(RAMP) to C_(HOLD) may be 2:1. The chargesharing operates to perform sample data filtering in which the filteringcharacteristics improve phase noise at the expense of transient responsesince the voltage change at each update will be lessened but the time toreach a desired voltage will increase. In this manner a control voltageindicative of the phase difference between one of the signals 1520 andthe divide-by-R output 1530 may be provided to one of the inputs 1514 ofthe VCO 400.

FIG. 24C illustrates a timing diagram for the phase detector/sample holdcircuit 1502. In FIG. 24C the operation of the charge switch 2302,V_(NOM) switch 2350 and sample/hold switch 2304 are shown in relation tothe f_(REF)/R output 1530 and the shift register outputs 1520[0, 1, . .. M]. As shown in FIG. 24C, the falling edges of the M+1 outputs of theshift register are each incrementally out of phase of the adjacentoutput. All of the signals in FIG. 24C except the shift register outputsare clocked by the reference clock 106 (f_(RE)F). The period of thereference clock is shown in the figure as T_(REF).

As discussed above, FIG. 22 shows a portion of the phase detector/samplehold circuit 1502 for performing the phase detection for one of theoutputs of the phase detector/sample hold circuit 1502. To perform themulti-line phase detection of FIG. 16, a plurality of the circuits1502[0] of FIG. 22 may be used as shown in FIG. 25. In the circuit ofFIG. 25, the resistors R1, R2, R3, and R4 transistors 2320, and gates2306, 2308, 2310, and 2312 shown in FIG. 22 have been combined incontrol blocks 2502 for ease of illustration. In operation, each of thecharge switches 2302 operates in unison and each of the sample/holdswitches 2304 operates in unison. Likewise all the SEL signals areapplied together. Thus, during one phase detection cycle, the phasedifference between the divide-by-R output signal 1530 and each of thefalling edges of the M+1 shift register outputs 1520 is detected andapplied to the phase detector/sample hold outputs 1512. Because fallingedges of each of the shift register output 1520[0, 1, . . . M] areincrementally out of phase with the adjacent output, each of the phasedetector/sample sample hold outputs 1512[0, 1, . . . M] will be atdifferent voltage level.

As mentioned above, each portion of the phase detector/sample holdcircuit 1502 may include a calibration switch, such as calibrationswitch 2340 or 2342 of FIG. 22. As shown in FIG. 25, half of the outputs1512 will connect to circuitry containing switch 2340 and half tocircuitry containing switch 2342. During digital or coarse tuning, theswitches 2340 will be closed to provide V_(NOM) to half of the outputsand the switches 2342 will be closed to provide ground to the other halfof the outputs. Thus, for example, during coarse tuning output 1512[0]may be provided V_(NOM) and output 1512[1] may be provided ground. Inoperation this allows the digital control to be performed atapproximately the center of the range of the analog variable capacitanceC_(A).

The generation of a plurality of control voltages for control of the VCO400 in the manner described above in which each voltage is offset fromthe adjacent control voltage is particularly advantageous when combinedwith the VCO circuit of FIGS. 4 and 17C. As shown in FIG. 17C, thecontinuously variable capacitance (C_(A)) 406 is formed from the sum ofthe individual capacitance circuits C_(A0), C_(A1), . . . C_(AM). Eachindividual capacitance circuit has a nonlinear relationship between thephase of f_(OUT) and the resulting capacitance of the one capacitancecircuit. However, utilizing a plurality of capacitance circuitscontrolled with voltages generated according to the techniques describedabove yields a relatively linear relationship between the phase and thetotal capacitance C_(A). The ability to obtain a relatively linearrelationship between the phase and the total capacitance C_(A) may beseen with reference to FIGS. 26-32.

FIG. 26 illustrates the capacitance vs. phase (phase of f_(out)) curvefor a single capacitance circuit, for example C_(A0). FIG. 27illustrates the nonlinear nature of the curve of FIG. 26 by plotting thetime derivative of the capacitance of C_(O) (i.e. the slope). This isthe quantity that directly determines loop performance. FIG. 28illustrates the individual capacitance vs. phase curves for a pluralityof capacitance cirucits C_(A0), C_(A1), C_(A2) . . . while FIG. 28Aillustrates the time derivative of the individual curves of FIG. 28. Ascan be seen from the figure, each successive capacitance curve is timeshifted from its adjacent curve. However, the total capacitance C_(A)406 is the summation of the individual capacitances. FIG. 29 and 30illustrate the capacitance vs. phase curves resulting from the summationof the individual capacitances. For example, FIG. 29 illustrates thetotal capacitance C_(A) for M+1=5. Similar, FIG. 30 illustrates thetotal capacitance C_(A) for M+1=20. The substantial improvements inlinearity of the total capacitance C_(A) can be seen more clearly withreference to the time derivative graphs shown in FIG. 31 for each caseM+1=20 and M+1=5. By comparing the curve of FIG. 27 with the curves ofFIG. 31, it can be seen that the use of multiple individual capacitancecircuits C_(A0), C_(A1), C_(A2), . . . C_(AM) provides a substantiallylinear circuit, with linearity improving as more circuits are utilized.In one embodiment 5 or more capacitance circuits may be utilized, morepreferrable 10 or more and most preferrable 20 or more. Thus, byutilizing multiple capacitance circuits a more linear relationshipbetween phase and capacitance may be obtained. Moreover because thecapacitance range of the analog fine tuning control is relativelylimited, e.g. 2% of the total capacitance, the amount of nonlinearitiesin the C_(A) to f_(OUT) relationship are not critical. Thus, thecircuits described herein provide a relatively linear relationshipbetween the detected phase differences in the PLL and f_(OUT).

As noted above, in one embodiment twenty control voltages 1514 may beutilized to provide the fine analog control for the VCO 400 for wirelesscommunications. Though the analog control has been described herein forillustrative purposes with references to single ended figures, it willbe recognized that the fully differential system such as shown in FIG.10 may be utilized. In such a system the twenty control voltages may beapplied to both C_(AN) and C_(AP) each formed from a plurality ofcapacitance circuits C_(AN0), C_(AN1), . . . C_(AN20) and C_(AP0),C_(AP1), . . . C_(AP20) respectively. In one wireless embodiment, thecomponent values for the individual capacitors C1 and C2 in each of thecapacitance circuits may be for each synthesizer RF1, RF2, and IF:

RF1: C1=0.04 pF and C2=0.12 pF

RF2: C1=0.06 pF and C2=0.18pF

IF: C1=0.13 pF and C2=0.39pF.

The resulting ranges for C_(AN) and C_(AP) are thus:

RF1: C_(AN)=C_(AP)=0.6 to 0.8 pf

RF2: C_(AN)=C_(AP)=0.9 to 1.2 pf

IF: C_(AN)=C_(AP)=1.95 to 2.6 pf.

Further, the transistors T₀, T₁, . . . T_(M) in the capacitance circuitsmay be sized for both the RF1 and RF2 synthesizers to have W/Ldimensions of 13/.35. Because the frequency range of the IF synthesizermay be large, the IF synthesizer transistors T₀, T₁, . . . T_(M) may beselectable depending upon the frequency that the IF synthesizer isoperating within. For frequencies 75 MHz or less the IF transistors maybe sized 1.625/0.35. For other frequency ranges the values may be:

75-150 MHz: 3.25/0.35 (2×)

150-300 MHz: 6.5/0.35 (4×)

300-600 MHz: 13/0.35 (4×).

In the exemplary embodiment, the individual circuit elements of thephase detector/sample hold circuit may have values of R1=500Ω, R2=500Ω,R3=1000Ω, R4=2000Ω, C_(RAMP)=40 pF, and C_(HOLD)=20 pF.

The techniques described herein for the fine tuning analog control ofthe PLL thus provide a system in which the frequency generation may beaccomplished through the use of a variation in a VCO capacitance.Furthermore, the techniques shown herein may be integrated into a singleintegrated circuit since capacitors of excessive size or traditionalvaractors are not required. Moreover, the transfer function of the PLLis relatively linear over the desired frequency range and relativelylarge capacitance changes within the VCO may be accomplished withoutlarge phase noise. In this manner a linear circuit behavior may beobtained without degrading phase noise performance

The benefits of the techniques disclosed herein may be obtained whileutilizing variations of the various circuits shown herein. For example,the use of multiple capacitive elements in the VCO under control of aplurality of control voltages may be accomplished with more traditionalPLL designs. Thus, for example a circuit such as shown in FIG. 32 may beutilized. As shown in FIG. 32, phase detector/charge pump/loop filtercircuitry 3200 may be utilized to generate a voltage V_(MASTER)indicative of the phase difference. The V_(MASTER) voltage may then beconverted to a plurality of voltage signals 3206[0], 3206[1] . . .3206[M] by a 1 to M+1 converter 3204. Each voltage signal 3206 mayinclude an incremental additional voltage so that 3206[0]=V_(MASTER),3206[1]=V_(MASTER)=Δv, 3206[0]=V_(MASTER)+Δ2v . . . In this manner,multiple voltage control signals may be provided to control a VCO havingvariable capacitive circuits similar to that shown in FIG. 22.

Though shown herein with respect to a voltage controlled oscillator, itwill be recognized that the concepts of the present invention may beutilized with other controlled oscillators. Thus, for example, thepresent invention may be utilized with a current controlled oscillator.Further, various circuits and techniques shown herein may be utilizedseparately or in combination without requiring the use of all circuitsand techniques shown herein. Thus, aspects or the digital control may beutilized independent of aspects of the analog control and vice-versa.Further, some concepts shown herein may be utilized in applicationsdifferent from the wireless communications embodiments discussed.

In addition, further modifications and alternative embodiments of thisinvention will be apparent to those skilled in the art in view of thisdescription. For example, the use of n-channel and p-channel devices andassociated logic levels are shown as example arrangements of devicetypes, and it will be recognized that the present invention is notlimited by these example arrangements. Accordingly, this description isto be construed as illustrative only and is for the purpose of teachingthose skilled in the art the manner of carrying out the invention. It isto be understood that the forms of the invention herein shown anddescribed are to be taken as the presently preferred embodiments.Various changes may be made in the shape, size and arrangement of parts.For example, equivalent elements may be substituted for thoseillustrated and described herein, and certain features of the inventionmay be utilized independently of the use of other features, all as wouldbe apparent to one skilled in the art after having the benefit of thisdescription of the invention.

I claim:
 1. A frequency synthesizer integrated circuit having at leastone variable frequency output, the output frequency varying as acapacitance within the synthesizer varies, comprises: a controllableoscillator integrated within the frequency synthesizer; a variablecapacitance circuit integrated within the frequency synthesizerintegrated circuit and operatively coupled to the controllableoscillator circuit, the variable capacitance circuit comprising, a firstcapacitor and a second capacitor coupled in series between a first nodeand a second node, a third node coupled between the first and secondcapacitors, and a variable resistance element coupled between the thirdnode and the second node, the first and second capacitors beingindependent of a capacitance of the variable resistance element, if suchcapacitance of the variable resistance element exists; and a variablecontrol signal coupled to the variable resistance element, thecontrollable oscillator being provided electrically between the variablefrequency output and the variable control signal, wherein changes in thevariable control signal result in changes in a capacitance between thefirst node and the second node of the variable capacitance circuit dueto changes in the resistance of the variable resistance element, thechanges in the resistance of the variable resistance element resultingfrom changes in the variable control signal that is coupled to thevariable resistance element.
 2. The frequency synthesizer of claim 1,wherein an output frequency of the controllable oscillator is controlledby controlling the variable control signal.
 3. The frequency synthesizerof claim 2, wherein the variable control signal is a voltage signal. 4.The frequency synthesizer of claim 2, further comprising a plurality ofthe variable capacitance circuits and a plurality of variable controlsignals, wherein each of the variable capacitance circuits isoperatively connected to at least one of the plurality of variablecontrol signals.
 5. The frequency synthesizer of claim 4, wherein theplurality of the variable capacitance circuits are connected inparallel.
 6. The frequency synthesizer of claim 5, wherein the pluralityof variable control signals is comprised of at least five variablecontrol signals.
 7. The frequency synthesizer of claim 1, furthercomprising a plurality of the variable resistance elements coupledbetween the third node and the second node.
 8. The frequency synthesizerof claim 1, wherein the variable resistance element is comprised of atransistor.
 9. A method of operating a frequency synthesizer integratedcircuit, comprising: providing a controllable oscillator integratedwithin the frequency synthesizer; providing a capacitance circuitintegrated within the frequency synthesizer integrated circuit andcoupling the capacitance circuit with the controllable oscillator, thecapacitance circuit comprising a first capacitor and a second capacitorcoupled in series between a first node and a second node, a third nodebeing between the first and second capacitors; providing a variableresistance element coupled to the third node; generating a variablecontrol signal; and controlling the resistance of the variableresistance element with the variable control signal to vary thecapacitance of the capacitance circuit, the first and second capacitorsbeing independent of a capacitance of the variable resistance element,if such capacitance of the variable resistance element exists; whereinchanges in the variable control signal result in changes in acapacitance between the first node and the second node of thecapacitance circuit due to changes in the resistance of the variableresistance element, the changes in the resistance of the variableresistance element resulting from changes in the variable control signalthat is coupled to the variable resistance element.
 10. The method ofoperating a frequency synthesizer of claim 9, wherein an outputfrequency of the controllable oscillator is controlled by varying thecapacitance of the capacitance circuit.
 11. The method of operating afrequency synthesizer of claim 10, further comprising the step ofproviding a plurality of variable capacitance circuits and generating aplurality of variable control signals, wherein each of the variablecapacitance circuits is operatively connected to at least one of theplurality of variable control signals.
 12. The method of operating afrequency synthesizer of claim 11, wherein the plurality of variablecontrol signals is comprised of at least five variable control signals.13. The method of operating a frequency synthesizer of claim 9, whereinthe variable resistance element is comprised of a transistor.
 14. Themethod of operating a frequency synthesizer of claim 9, furthercomprising the steps of providing a plurality of variable resistanceelements coupled to the third node, wherein the capacitance of thecapacitance circuit is varied by controlling the resistance of theplurality of variable resistance elements.
 15. The method of operating afrequency synthesizer of claim 14, wherein each of the plurality ofvariable resistance elements is comprised of a transistor.
 16. Afrequency synthesizer having at least one variable frequency output, theoutput frequency varying as a capacitance within the synthesizer varies,comprising: a controllable oscillator integrated within the frequencysynthesizer; a variable capacitance circuit integrated within thefrequency synthesizer, the variable capacitance circuit coupled to thecontrollable oscillator, the variable capacitance circuit comprising, afirst capacitor and a second capacitor coupled in series between a firstnode and a second node, a third node coupled between the first andsecond nodes, and a variable resistance element coupled between thethird node and the second node, the variable resistance element being inparallel with the second capacitor, the second capacitor beingindependent of a capacitance of the variable resistance element, if suchcapacitance of the variable resistance element exists; and an analogvariable control signal coupled to the variable resistance element, thecontrollable oscillator being provided electrically between the variablefrequency output and the analog variable control signal, wherein changesin the analog variable control signal result in changes in a capacitancebetween the first node and the second node of the variable capacitancecircuit due to changes in the resistance of the variable resistanceelement, the changes in the resistance of the variable resistanceelement resulting from changes in the analog variable control signalthat is coupled to the variable resistance element.
 17. The frequencysynthesizer of claim 16, wherein in output frequency of the controllableoscillator is controlled by controlling the variable control signal. 18.The frequency synthesizer of claim 17, wherein the variable controlsignal is a voltage signal.
 19. The frequency synthesizer of claim 17,further comprising a plurality of the variable capacitance circuits anda plurality of the variable control signals, wherein each of thevariable capacitance circuits is operatively connected to at least oneof the plurality of variable control signals.
 20. The frequencysynthesizer of claim 19, wherein the plurality of the variablecapacitance circuits are connected in parallel.
 21. The frequencysynthesizer of claim 19, wherein the plurality of variable controlsignals is comprised of at least five variable control signals.
 22. Thefrequency synthesizer of claim 16, further comprising a plurality of thevariable resistance elements coupled between the third node and thesecond node.
 23. The frequency synthesizer of claim 16, wherein thevariable resistance element is comprised of a transistor.
 24. A methodof operating a frequency synthesizer integrated circuit, comprising:providing a controllable oscillator integrated within the frequencysysnthesizer; providing a capacitance circuit integrated within thefrequency synthesizer integrated circuit, the capacitance circuitcoupled to the controllable oscillator, the capacitance circuitcomprising a first capacitor and a second capacitor coupled in seriesbetween a first node and a second node, a third node being between thefirst and second nodes; providing a variable resistance element coupledto the third node, the variable resistance element being in parallelwith the second capacitor, the second capacitor being independent of acapacitance of the variable resistance element, if such capacitance ofthe variable resistance element exits; generating an analog variablecontrol signal; and controlling the resistance of the variableresistance element with the variable control signal to vary thecapacitance of the capacitance circuit, wherein changes in the variablecontrol signal result in changes in a capacitance between the first nodeand the second node of the capacitance circuit due to changes in theresistance of the variable resistance element, the changes in theresistance of the variable resistance element resulting from changes inthe analog variable control signal that is coupled to the variableresistance element.
 25. The method of operating a frequency synthesizerof claim 24, wherein an output frequency of the controllable oscillatoris controlled by varying the capacitance of the capacitance circuit. 26.The method of operating a frequency synthesizer of claim 25, furthercomprising the step of providing a plurality of capacitance circuits andgenerating a plurality of variable control signals, wherein each of thecapacitance circuits is operatively connected to at least one of theplurality of variable control signals.
 27. The method of operating afrequency synthesizer of claim 24, wherein the variable resistanceelement is comprised of a transistor.
 28. The method of operating afrequency synthesizer of claim 24, further comprising the steps ofproviding a plurality of variable resistance elements coupled to thethird node, wherein the capacitance of the capacitance circuit is variedby controlling the resistance of the plurality of variable resistanceelements.
 29. The method of operating a frequency synthesizer of claim28, wherein each of the plurality of variable resistance elements iscomprised of a transistor.
 30. A frequency synthesizer having at leastone variable frequency output, the output frequency varying as acapacitance within the synthesizer varies, comprising: at least one ormore variable control signals, the variable control signals being basedat least in part upon the output frequency of the frequency synthesizer;and a substantially linearly variable capacitance circuit integratedwithin the frequency synthesizer, the variable capacitance circuitcomprising, a plurality of nonlinear capacitance circuits integratedwithin the substantially linearly variable capacitance circuit, at leastsome of the nonlinear capacitance circuits controlled by the at leastone or more variable control signals, wherein capacitances of theplurality of nonlinear capacitance circuits combine to form acapacitance within the substantially linearly variable capacitancecircuit and wherein the nonlinear capacitance circuits are variablebetween a plurality of capacitance amounts including at least twonon-zero capacitance amounts; wherein the substantially linearlyvariable capacitance circuit has a substantially linear relationshipbetween a signal derived from the frequency output and the capacitanceof the linear variable capacitance circuit, even though thesubstantially linearly variable capacitance circuit includes theplurality of nonlinear capacitance circuits.
 31. The frequencysynthesizer of claim 30, wherein the derived signal is a signalindicative of the phase of the frequency output.
 32. The frequencysynthesizer of claim 30, wherein each of the plurality of nonlinearcapacitance circuits further comprises: a first capacitor coupledbetween a first node and a second node; a third node coupled between thefirst and second nodes, and a variable resistance element coupledbetween the third node and the second node.
 33. The frequencysynthesizer of claim 32, wherein each of the plurality of nonlinearcapacitance circuits further comprises: a second capacitor coupledbetween a first node and a second node, the third node being coupledbetween the first capacitor and the second capacitor.
 34. The frequencysynthesizer of claim 30, further comprising a controllable oscillatoroperatively connected to the nonlinear capacitance circuits, wherein anoutput frequency of the controllable oscillator is controlled bycontrolling the nonlinear capacitance circuits.
 35. A method ofoperating a frequency synthesizer, comprising: providing integratedvariable capacitance circuitry having a substantially linearrelationship between a signal derived from a frequency output of thefrequency synthesizer and the capacitance of the variable capacitancecircuitry, the variable capacitance circuitry formed from a plurality ofindividual capacitance circuits; providing a plurality of variablecontrol signals based at least in part upon an output frequency of thefrequency synthesizer, the variable control signals configured tocontrol a capacitance of each of the plurality of individual capacitancecircuits in a nonlinear manner, the nonlinear capacitance circuits beingvariable between a plurality of capacitance amounts including at leasttwo non-zero capacitance amounts; and adjusting the variable controlsignals to vary a total capacitance of the variable capacitancecircuitry in a substantially linear manner, even though the individualcapacitance circuits have capacitances that vary in a nonlinear manner.36. The method of operating a frequency synthesizer of claim 35, whereinthe step of providing an integrated variable capacitance circuitryfurther comprises the steps of: for each of the plurality of capacitancecircuits, providing a first capacitor and a second capacitor coupled inseries between a first node and a second node, providing a third nodebetween the first and second capacitors; for each of the plurality ofcapacitance circuits, providing a variable resistance element coupled tothe third node; for each of the plurality of capacitance circuits,generating a variable control signal; and for each of the plurality ofcapacitance circuits, controlling the resistance of the variableresistance element with the variable control signal to vary thecapacitance of the capacitance circuit.
 37. The method of operating afrequency synthesizer of claim 35, further comprising the steps ofproviding a controllable oscillator operatively connected to theintegrated variable capacitance circuitry, and controlling an outputfrequency of the controllable oscillator by controlling the capacitancecircuits.
 38. The frequency synthesizer of claim 1, the variable controlsignal being an analog control signal.
 39. The method of claim 9, thevariable control signal being an analog control signal.